Implemented multiple payment handling on my side/evening project! Notes:
Stripe: Super easy. Look at that, I'm taking credit cards!
PayPal: Ah PayPal, my old friend! Just need double up on IPN and return variable verification in case either barfs.
Amazon: Initial form needs a signature. Well, add a roundtrip to the server based on form changes. Usual server verification stuff. Done!
Google Checkout: Pass order details to server. Create XML payload with cryptographic signature, submit to Google's servers on the backend. Get redirect URL for individual user. Pass custom URL back to browser to move user to actual checkout process. Catch return data, verify with server....what the fuck, Google. Lighten up. Nobody uses Google Checkout anyway, I give up.