Hey there!

As said in the docs https://itch.io/docs/creators/... itch.io does support paypal, amazon and stripe (and bitcoin) for buying games when they have a price. But how about free browser games with ingame shops for loot chests or other equipment (which can be farmed, also!).

Thanks and good day :)

Hello, right now we don't support in-game purchases, but it's something we'd like to add in the future. Thanks

Thanks for the answer!

Is it allowed to directly use paypal or other payment provider inside a game which is distributed via itch.io?

You're free to use an existing payment provider in your game, just make sure it's not too obtrusive or it might get rated down.