In Steam, on the bottom left, you can choose "Add A Game" > "Add a Non-Steam Game" and choose an EXE on your computer, so you can have any game you download from itch imported into and playable from Steam.
When you're checking out on itch.io, you'll see a screen with two options at the bottom: "Pay with Paypal" and "Pay with card". If you choose "Pay with card", it'll be processed by Stripe instead of paypal.