XFlow vs Back In Stock! Alert Engine: which back in stock app fits your Shopify store?

A sold-out product page is one of the quietest leaks in a Shopify store. The shopper arrived, wanted the item, and left with nothing to do. A back in stock app turns that dead end into a waitlist: a "Notify me" button collects an email (sometimes a phone number), and the app writes to that person the moment inventory comes back.

The big names in this category have thousands of reviews and are covered everywhere. XFlow Back in Stock and Back In Stock! Alert Engine are not in that group. Both are recent, both are rated 4.9 out of 5, and both do the same core job. Yet they are built on two very different ideas of what a restock tool should be, and above all they bill in two very different ways. This page goes through everything we could verify on their App Store listings and documentation, and says clearly where information is missing.

Who makes these two apps

XFlow Back in Stock

XFlow is listed on the App Store under the developer name XFlow, with an address in Hanoi, Vietnam, and a website at xflow.so. Its documentation lives on the FoxEcom help center, and FoxEcom's own migration guide explains that XFlow is the replacement for FoxKit Back in Stock, the restock module of the FoxKit app, which was fully deactivated on June 30, 2026. In other words, XFlow is a new listing (launched December 1, 2025) built by a team that already ran a restock feature for FoxKit merchants.

At the time of checking, the listing showed 4.9 stars from 70 reviews: 69 five-star and 1 one-star. The listing title has changed a few times ("Back in Stock Alert", "Back in Stock Automation", "Back in Stock Alerts"), which is worth knowing if you search for it. The app interface is available in English, Italian, German, French and Spanish.

The positioning is clear from the listing: XFlow presents itself as an automation tool, with a visual workflow builder, several restock campaigns running at the same time, follow-up emails, web push and SMS.

Back In Stock! Alert Engine

Alert Engine is published by Script Engine, a Shopify development agency based in Surat, India. Besides client work (store builds, custom apps, migrations), Script Engine publishes a wishlist app, SE Wishlist Engine, and a checkout app. Alert Engine was launched on December 23, 2024.

The listing showed 4.9 stars from 26 reviews when we checked (a single one-star review, the rest five stars). The app interface is listed in English only.

Alert Engine's angle is different: it is an email-only alert tool that covers three triggers instead of one. Besides restock alerts, it can send low-stock alerts and price-drop alerts, and it syncs subscribers to Klaviyo and Omnisend on its top plan.

Pricing: flat plans against revenue-based billing

This is the biggest difference between the two apps, so it comes first.

Alert Engine: four flat plans

All prices are in US dollars and billed through Shopify every 30 days, with a 7-day free trial on paid plans and a 17% discount on yearly billing.

• Free: 50 email alerts, unlimited subscribers, restock alerts only, custom button and templates, import and export of subscribers, open and delivery logs.
• Basic, $4/month ($40/year): unlimited email alerts, restock alerts, insights.
• Advanced, $8/month ($80/year): adds low-stock alerts.
• Premium, $12/month ($120/year): adds price-drop alerts, the button on collection pages, Klaviyo and Omnisend integrations, sending from your own domain, multi-language and multi-location inventory.

The bill never moves with your sales. A store that recovers $200 or $20,000 in a month pays the same.

XFlow: a fee tied to attributed revenue

XFlow's plans are also in US dollars, with a 7-day free trial on paid plans.

• Free, $0: 5 attributed orders, 1 live restock campaign, unlimited back in stock emails, the visual workflow builder, branded templates, a custom Notify button on the product page, double opt-in, onboarding and migration help, 24/7 support.
• Starter, $9/month: includes $1,000 of attributed revenue per month, then $7 per additional $1,000. Adds unlimited campaigns, web push, SMS and advanced analytics.
• Growth, $19/month: includes $3,000 of attributed revenue per month, then $5 per additional $1,000. Adds the Notify button on any page, a branded sender address, multi-language emails with AI translation and Shopify Flow.

"Attributed revenue" means orders that XFlow links to its restock campaigns. The documentation explains that the analytics report attributed orders and revenue, but the listing does not state the attribution window, and it does not say what happens on the free plan after the fifth attributed order. Ask support before you rely on the free plan for a large catalog.

A quick reading of the listed rates, at round numbers:

The logic is simple: XFlow costs more when it works well, because you are paying on results. Alert Engine is the cheaper tool as soon as recovered sales pass a few thousand dollars a month. On the other hand, if a store recovers very little, XFlow's free plan sends unlimited emails, while Alert Engine's free plan caps out at 50 alerts.

Criterion by criterion

Alert channels

Both apps send back in stock emails automatically when inventory returns, and both also allow manual and batch sends, which helps when a big restock lands and you want to control timing.

XFlow goes further on channels. Web push and SMS are included from the Starter plan, with SMS limited to "select countries" according to the listing. Alert Engine's listing and website only mention email, so if SMS or push matter to you, Alert Engine is not the right tool today.

Types of alerts

Here the advantage flips. Alert Engine covers three triggers:

• restock alerts on every plan,
• low-stock alerts from the Advanced plan ($8),
• price-drop alerts on the Premium plan ($12).

XFlow's listing only describes back in stock flows. Low-stock and price-drop alerts are not listed, so we count them as missing. If you want to tell waitlisted shoppers that a product is on sale, or create urgency when only a few units remain, Alert Engine does it natively. For the urgency side, our article on low stock alerts and what actually converts goes into the details.

The Notify me button

Both apps add a customizable Notify me button to sold-out products. XFlow's listing mentions placements on the product page, product cards, quick view and other page types, with "any page" placement unlocked on Growth ($19). Alert Engine shows the button on product pages on every plan and adds collection pages on Premium ($12).

XFlow includes double opt-in on the free plan, which is useful for stores that want a clean list and a clear consent trail. Double opt-in is not listed on Alert Engine's page.

Automation and campaigns

This is XFlow's core. Its documentation describes workflows made of triggers, conditions and actions, reusable email templates with dynamic product and customer fields, and campaigns that group products, dates and a priority level when several campaigns overlap. You can run one campaign for a best-seller and another for a seasonal collection, pause one without touching the other, and add follow-ups (the docs give the example of a reminder 24 hours after the first restock email). The listing also mentions best-time sending and discounts inside restock flows.

Alert Engine works more like a switchboard: you turn alerts on, edit the templates, set the sender name and reply-to address, and the app sends. The App Store tags mention follow-up emails and discounts, but neither the listing bullets nor the app's website describe how they work, so we score them as partial. If you want sequences and segmentation, XFlow is clearly ahead.

Analytics

XFlow's listing describes an analytics dashboard that ranks demand, helps plan restocks and tracks recovered sales, with advanced reports from Starter. Alert Engine includes open, delivery and read logs on every plan, including the free one, and an "Insights" view on paid plans. Both give you a sense of demand per product. XFlow is more focused on revenue attribution, Alert Engine on email delivery logs.

Integrations

Alert Engine's Premium plan syncs subscribers to Klaviyo and Omnisend, so a restock signup can feed your main email tool. XFlow's listing does not mention Klaviyo or Omnisend. It does work with Shopify Flow on Growth, which opens the door to custom automations inside Shopify itself.

On the page builder side, Alert Engine lists PageFly; XFlow lists PageFly, GemPages, EComposer, Shogun and Foxify.

Languages and sending domain

Both apps can send alerts in several languages and from your own domain, but on different plans: XFlow on Growth ($19), Alert Engine on Premium ($12). XFlow adds AI translation of emails. The app interface itself is available in five languages for XFlow and in English only for Alert Engine.

Multi-location inventory

Alert Engine's Premium plan lists multi-location inventory support. XFlow's listing does not mention it. If you ship from several warehouses or have stores and an online stock, check this point with XFlow's support before installing.

Data portability

Alert Engine includes subscriber import and export on every plan, which is reassuring: your waitlist is never locked in. XFlow's documentation describes an import screen for subscriptions (used for the FoxKit migration) and the free plan lists "onboarding and migration", but a general export is not described on the listing.

Support and track record

Both listings advertise 24/7 support. XFlow benefits from a team that already ran a restock feature inside FoxKit, and from a larger review count. Alert Engine has fewer reviews but they are almost all five stars, and recent reviews praise how fast the team answers.

Three concrete store profiles

A small fashion store with occasional restocks

A one-person clothing store restocks a dozen products a month and recovers maybe $300 in sales from waitlists. Alert Engine's Basic plan at $4 a month removes the 50-email cap and does the job. If the owner also wants a low-stock message on the last sizes, Advanced at $8 adds it. XFlow's free plan could also work here, but the 5 attributed orders limit is low, and paying $9 a month for $1,000 of included revenue gives little extra for a store this size.

A growing brand with weekly drops

A streetwear or sneaker brand sells out every week, gets hundreds of waitlist signups per product, and wants a first alert, a follow-up 24 hours later, an SMS for its best customers and a separate campaign per drop. That is exactly what XFlow's workflow builder and multi-campaign setup are designed for. The bill will grow with recovered revenue, but so will the margin. Growth becomes cheaper than Starter once attributed revenue passes about $2,500 a month.

A Klaviyo-centric store with two warehouses

A home goods store runs all its email marketing in Klaviyo and stocks products in two locations. Alert Engine's Premium plan at $12 a month syncs restock signups to Klaviyo, handles multi-location inventory, adds price-drop alerts and sends from the store's domain. XFlow's listing mentions neither Klaviyo nor multi-location support, so Alert Engine is the safer pick here.

Migrating from one app to the other

From Alert Engine to XFlow

Export your subscribers from Alert Engine (available on every plan). XFlow's free plan lists onboarding and migration, so contact its support with the file before you switch. Rebuild a simple one-email workflow first, test it on a product you can toggle out of stock, then add follow-ups. Remove Alert Engine's button only once XFlow's button appears correctly, to avoid a gap where shoppers cannot sign up.

From XFlow to Alert Engine

Alert Engine's import works on every plan. Ask XFlow's support for an export of pending subscriptions, since a self-serve export is not described on the listing. Recreate your templates in Alert Engine, check the button styling on your theme, and pick Advanced or Premium if you want low-stock or price-drop alerts.

In both directions, the main risk is losing pending signups. Keep the old app installed until the import is confirmed.

Our take

These two apps share a rating and a purpose but not a philosophy. Alert Engine is a low-cost, predictable tool that covers more alert types and plugs into Klaviyo and Omnisend. XFlow is an automation tool with more channels and more control, billed on what it brings back. If you also sell items before they exist, our comparison of Bat Preorders and PreProduct covers the pre-order side of the same problem.