The AfterSell checkout tab is made possible by the Checkout Extensibility update by Shopify. This update allows merchants to insert app blocks into their Checkout rather than needing to write custom scripts.
Why is the AfterSell Checkout Tab only available to Shopify Plus merchants?
The Checkout Extensibility update by Shopify is only available to Shopify Plus merchants.
Unfortunately, this means that merchants on other Shopify plans, such as Basic or Advanced, do not have access to the AfterSell checkout tab. This is a limitation put in place by Shopify, and AfterSell does not have a say in it.
If you are interested in upgrading to the Shopify Plus plan to access the AfterSell checkout tab and other exclusive features, please reach out to Shopify's support team for more information and they'd be more than happy to help with that!
Conclusion
Shopify's Checkout Extensibility is required for AfterSell Checkout, and therefore its usage is currently only available to Shopify Plus Merchants.