How can I add a custom currency to commerce? I'm trying to add a custom currency (like points) but I can only choose the "real world" currency. Can I change the currency in commerce or I have to do it with a module? If so, which module should I use? Thank you !
PS: I have to do it in commerce, is for a school project
Have a look at the Commerce Userpoints module. Some details from its project page:

It allows users to create commerce_line items with negative points value, behaving like a discount system in the order process.
The user can choose how many points he wants to use, they will be automatically taken from his userpoints account.