Hi, I’m considering the addition of in-app purchases in my FOSS game. The PlayStore version will use the Android Billing library, as it is the law (of Google), but this can’t be used in F-Droid as it is not open source.
Which services would you suggest as a replacement? Is there any app in F-Droid with in-app purchases?
There’s none because it needs a FOSS payment lib.
Maybe try to integrate it in your website? Pay for a key? Etc
But, since it’s FOSS… anyone can compile the “full” version, by design.
Yes, I’m aware of the properties of the FOSS 
I just wish the published packages will respect the intended design. Ideally I would want the app on F-Droid to provide the same experience than the app on the PlayStore. For example, if I implement items that are unlocked by playing or via purchases (if the player wants to take shortcuts), I’d like the purchases to be available on F-Droid too.
Stripe seems to be nice, but it’s not open source.
Lago is open source, but it seems that it is a solution to bill the usage of the app, not consumable items.
Thus the question!
sure 
F-Droid might not allow it in the app, but you are free to host whatever in your site
If his game is an online multiplayer game, he can make it so purchased items are stored for each account on his server. And those purchased items can only be used and will be shown to other players when the game is played on his server.
The point of buying items in a game is bragging about them in front of people. Right ?