Back-in-stock notification
The 'email me when this is back' signup that replaces add-to-cart on a sold-out size, then fires the moment that variant is restocked.
See it
What it is
When a variant sells out, the add-to-cart button is dead space. A back-in-stock notification puts a signup there instead: the shopper leaves an email or phone number, you store the request against that exact variant, and when inventory for it crosses from zero back to positive, an automated message goes out. These are consistently the highest-converting emails an ecommerce store sends, because the intent was captured at the exact moment of wanting.
Bind the signup to the variant, not the product. Someone who wanted the medium in olive does not want an email saying the XXL in beige returned, and product-level alerts train people to ignore you. Klaviyo, Shopify's built-in alerts, and apps like Back in Stock and Swym all model it this way.
The gotcha is the stampede. Blast 800 people at once about 40 units and 760 of them click through to a sold-out page and feel worse than before. Send in waves sized to the actual restock quantity, send oldest signups first, put a short reservation window or a direct add-to-cart link in the email, and cap how many alerts go out per unit. Also remember consent: an alert signup is a transactional request, so do not silently fold those addresses into your marketing list without a separate opt-in.
Ask AI for it
On the product detail page, when the selected variant has zero inventory, replace the add-to-cart button with a 'Notify me when available' button that opens an inline form with an email field (SMS optional), a marketing opt-in checkbox that is separate and unchecked, and a confirmation state reading 'We'll email you when the medium in olive is back'. Store each request against the variant ID, not the product ID. When inventory for that variant goes from 0 to positive, send alerts in waves sized to the restocked quantity, oldest signups first, with a deep link straight to that variant in the cart. Mark each request fulfilled after sending and expire stale requests after 90 days.