Currency Shop to buy

So I am trying to make one of these shops where you can buy the currency from a dev product. I am unsure totaly on how to do it but looked this up and it seems I need to use the thing which is linked below. I am unsure and confused on how to use it so if someone could expalin in what I got to do that would be great,

https://developer.roblox.com/en-us/api-reference/callback/MarketplaceService/ProcessReceipt