How do I make developer products in a script?

I want to make it so I can have someone join a purchasing hub and can buy multiple sets of items at once. But I have no idea how to do this, the only way I can think of is probably some external service.

I have seen this done in other games. Examples: Centrea - Roblox

In the screenshot above I purchased 3 products in one go at their hub.

I just need a base idea of how this is done. If you have any more questions about what I am trying to achieve just ask!