Sell add-ons on top of any plan

Declare a capability in code and price it from the admin panel, with an icon, screenshots, categories and its own limits. One add-on can require another, so the catalogue holds it back until the account has the one it builds on. Whether an account has it is one question your code asks, whatever way it was obtained. Its name and descriptions show in each customer's language, translated like the rest of your product.