A back-in-stock alert is a signup form on a sold-out product page that collects a shopper's email, phone or WhatsApp number and notifies them the moment you restock. It exists because out-of-stock traffic does not stop arriving — those visitors wanted the product enough to find it, and a greyed-out button converts that into nothing. This guide covers where the widget goes, which channel to notify on, how to time the send so you do not create disappointed customers, and when a preorder is the better answer.
The demand you are already throwing away
Every sold-out product page you own is still being found — through search, through ads you paid for, through links that were shared before the stock ran down. The visitor arrives having already done the hard part. They wanted it, they found it, and they are ready.
The default Shopify behaviour at that point is a disabled Add to Cart button and the word "Sold out". The shopper leaves. Usually they leave and buy the same thing somewhere else, which means the stockout has cost you the sale and handed a competitor a customer who might have stayed with you.
A signup form changes the outcome twice over. It captures a buyer whose intent is proven rather than assumed, so the eventual restock email lands on someone far more likely to convert than any cold list. And it turns your out-of-stock pages into a demand-forecasting instrument. Sixty people waiting on one size tells you something no sales report can, because a sales report can only measure demand you were able to fulfil.
That second effect is the one merchants underrate. The alerts recover revenue this month. The waitlist data stops you under-ordering the same product next quarter.
Where the signup widget goes
The obvious placement is the product page, replacing or sitting beneath the disabled Add to Cart button. That is where intent is highest and where most apps stop.
The placement people forget is the collection page. A shopper scanning a grid sees "Sold out" on a thumbnail and moves on without ever opening the product. A Notify Me control on the card captures that person before they have decided you have nothing for them.
Both matter, and they capture different people. Product-page signups come from shoppers who were already committed to that item. Collection-page signups come from shoppers who were still browsing — a wider, shallower group, but a group you otherwise lose silently.
Decide separately what the form asks for. Email only is the lowest-friction option and the right default. Asking for a phone number as well raises the value of each signup and lowers the number of them; whether that trade is worth it depends entirely on the next section.
What the button should say
The control replacing Add to Cart is doing a harder job than it looks. It has to make a promise the shopper believes, in about four words, at the exact moment they have been told no.
"Notify me when available" outperforms "Email me" because it names the event rather than the mechanism. "Join the waitlist" works on scarce or hyped items where being on a list is itself a signal of desirability, and works badly on ordinary restocks where it sounds like a queue.
Two details do more than the wording:
- Confirm immediately, in place. A shopper who submits and sees nothing change assumes it failed and leaves. Swap the form for a plain confirmation on the page.
- Say when, honestly. "We will email you the moment it is back" is a promise you can keep. "Back in 2 weeks" is one you probably cannot.
Resist adding a marketing opt-in checkbox to the form. It measurably reduces signups, and a restock subscriber who later buys is a far better route onto your list than one who was nudged into it while trying to do something else.
Email, SMS or WhatsApp: the channel decides the conversion
Restock alerts are time-sensitive in a way that most ecommerce email is not. A limited restock can be gone within the hour. An email that sits unopened until the evening commute is a notification you paid to send and nobody acted on.
Which channel wins is a question about your customers, not about the technology.
- Email is the sensible default. It costs effectively nothing per send, it carries images and multiple products comfortably, and every customer has one. It is also the slowest to be seen.
- SMS is opened within minutes. It suits high-value items and genuinely scarce restocks, where the message is worth a couple of cents and the speed changes the outcome. It is intrusive enough that using it for routine restocks will cost you subscribers.
- WhatsApp is the one most apps skip entirely, and it is the difference between reaching and missing your customers across India, Brazil, Indonesia, the UAE and much of southern Europe. In those markets a marketing email is close to invisible while a WhatsApp message is read like a message from a person.
If a meaningful share of your customers are in a WhatsApp-first market, that single fact should probably decide which app you install. Most back-in-stock apps on the Shopify App Store offer email and SMS; far fewer offer WhatsApp.
The practical setup for most stores is email as the baseline for everything, with SMS or WhatsApp reserved for restocks where speed genuinely matters — limited runs, popular sizes, anything you expect to sell through quickly.
Timing: the mistake that creates 370 disappointed customers
Here is the failure mode nobody warns you about. Four hundred people join the waitlist for a jacket. You restock thirty units. The app notifies all four hundred at once. Thirty people buy, and three hundred and seventy click through to a sold-out page for the second time.
You have now converted a group of interested customers into a group of annoyed ones, and taught them that your restock emails are not worth opening.
There are two sane responses:
- Quantity-matched notification. Alert only as many subscribers as you have units, in the order they joined. The rest stay on the list for the next restock. Everyone who gets an email can actually buy.
- Batched sending. Notify in waves — say fifty at a time, with a gap between waves — and stop when the stock is gone. This recovers more revenue than quantity-matching if some recipients do not act, at the cost of some later waves finding it sold out.
Quantity-matched is the safer default and the one to use on anything scarce. Batching suits larger restocks where you expect a low click rate and want to be sure the stock actually clears.
Either way, send immediately on restock rather than on a schedule. The value of a restock alert decays fast, and a daily digest is the wrong shape for this message.
When a preorder beats an alert
An alert and a preorder answer the same situation differently. The alert says "we will tell you when it is back". The preorder says "pay now, we will ship when it arrives".
The deciding factor is how confident you are in the restock date.
- Confident date, stock already on a purchase order? Take the preorder. You capture the money now, you learn real demand before the shipment lands, and the customer does not have to remember you exist in three weeks.
- Vague date, or no reorder decided? Collect the email. Promising a ship date you cannot hold is how you earn chargebacks and one-star reviews.
A preorder also changes what the page has to say. A customer paying today for something shipping later needs the expected date, the refund policy and an honest explanation on the page — not buried in terms. Stores that hide the delay have worse outcomes than stores that state it plainly.
In practice most catalogues want both, chosen per product: preorders on items with a confirmed inbound shipment, alerts on everything else.
Price drop alerts: the waitlist nobody sets up
Back-in-stock alerts capture shoppers blocked by availability. There is a second, larger group blocked by price — people who looked, wanted it, and decided not at that number.
A price drop alert lets them ask to be told when it changes. It costs nothing extra to run alongside restock alerts, it uses the same subscriber infrastructure, and it fires exactly when you are running a promotion and want traffic.
It also gives you something useful even when nobody buys: a count of how many people are waiting on a price, per product. That is a cleaner signal about whether an item is mispriced than any amount of guessing.
The same discipline applies as with restock alerts. Do not fire a price drop alert for a trivial change; a 3% cut is not news, and sending it teaches people to ignore you.
Serving customers who do not read English
If you sell internationally, the notification is the one touchpoint most likely to arrive in the wrong language. Your storefront may be translated. Your restock email frequently is not, because it is generated by an app rather than your theme.
Two things are worth checking before you commit to an app:
- Does it send the notification in the language the customer was browsing in, automatically, or does it need a separate template per language maintained by hand?
- Does that apply to SMS and WhatsApp as well as email? Multi-language support that covers only email is common and only half useful.
Stok handles 29 languages and picks the language from the customer's context rather than asking you to maintain parallel templates.
Feeding the waitlist into your email platform
A restock waitlist is a list of people who told you exactly which product they want. Leaving that inside the app, visible only as a number, wastes most of its value.
Synced into Klaviyo or whatever you send from, each subscriber arrives tagged with the product they are waiting on. That opens up flows the app itself will not run for you:
- A nurture sequence for someone waiting on a product you have decided not to reorder, pointing at the closest alternative.
- Segmenting a launch email by the people who waited on last season's version.
- Suppressing a general promotion for a product from everyone currently waiting on it, so the discount email does not arrive before the in-stock email.
The integration to check for is whether subscribers sync with the product reference attached, not merely as email addresses. A sync that hands over a flat list of addresses has thrown away the only thing that made the list interesting.
It is also worth deciding deliberately whether restock subscribers join your general marketing list. Legally that depends on your jurisdiction and how the form is worded. Practically, someone who asked to hear about one product has not asked to hear about everything, and treating those as the same consent is how a good list goes stale.
What to measure
Most stores measure signups, which is the least interesting number available. Signups tell you the widget is visible. They do not tell you whether the programme works.
The numbers worth watching:
- Recovered revenue — orders traced back to a restock notification. This is the only figure that answers "is this worth running".
- Notification-to-purchase rate — of the people you alerted, how many bought. If this is low, look at your timing before you look at your copy.
- Signups per out-of-stock product — ranks what to reorder, and by how much.
- Waitlist age — how long people have been waiting. A long-stale waitlist is either a product you should reorder or one you should stop listing.
Back-in-stock emails routinely see open rates well above standard marketing email, for the obvious reason that the recipient asked for this specific message about this specific product. If yours do not, the problem is usually that the send was late or the product was already gone again.
Setting it up
The whole setup is a short job on most themes. With Stok the sequence is:
- Install. The Notify Me button appears automatically on out-of-stock products — no theme code, no snippet to paste. It works on product and collection pages.
- Decide what the form collects. Email only to start. Add phone or WhatsApp if your market justifies it.
- Set the notification rule. Quantity-matched for scarce items, batched for large restocks.
- Turn on price drop alerts. Same widget, second reason to subscribe, no extra cost.
- Add preorders where you have a confirmed date. Per product, not globally.
- Check the email template. One product, one clear link, the price, and nothing else competing for the click.
The free plan covers unlimited subscriber collection and 20 email notifications a month, which is enough to see whether the demand is there before you pay for anything. Email, SMS and WhatsApp channels start on the $10 plan.
Mistakes worth avoiding
- Hiding sold-out products. Hiding the page removes the ranking, the traffic and the signup opportunity at once. Keep it live with a signup form.
- Alerting everyone at once on a small restock. Covered above, and it is the most common one.
- Sending a digest. A restock alert bundled with four other products is a newsletter, and it converts like one.
- Letting the waitlist rot. If you decide not to reorder, tell the people waiting and offer them something else. Silence is worse than a no.
- Ignoring collection pages. Half your potential signups never open the product page.
Choosing between apps
The Shopify App Store has a dozen back-in-stock apps and their feature lists look nearly identical. Feature count is not what separates them. Four questions are:
- Which channels, and at what price? Email is universal. SMS is common. WhatsApp is rare, and decisive if your customers are in a WhatsApp-first market. Check the per-message cost, not just the monthly fee — that is where the real bill comes from at volume.
- How does it decide who to notify? An app that only does "notify everyone" will eventually create the 370-disappointed-customers problem for you. Quantity-matched or batched sending is not a luxury feature.
- Does multi-language cover every channel? Email-only translation is common and half useful.
- Does the free plan let you find out? You cannot tell whether restock demand exists in your catalogue until you have collected signups for a month. An app that requires payment before you can measure that is asking you to guess.
Preorders are the other axis. Some apps treat them as a separate product with a separate subscription; others include them. If you expect to use both, count that cost in.
Where to go next
If you are choosing between apps, the comparison that matters is channels, language handling and how the app decides who to notify — not feature-count. If you are deciding between preorders and alerts, decide per product, using the restock date as the test.
You can see how Stok handles all of this, including WhatsApp and 29-language notifications, on the free plan.
