I think this is a great idea, despite its obvious flaw.

To make it a better offering, I would probably add sample custom payment gateway "proxies" in common languages (i.e. PHP, Ruby, etc.) for security.

What I mean by proxy is provide some generic server-side code as a middle man for the payment gateway which can confirm the details of an order (on your server) haven't been tampered with before forwarding onto the actual gateway (PayPal, etc.)

This way, the shops who don't mind dealing with dodgy purchases can run 100% client-side, while more others can use this feature to prevent invalid orders being created.

Am I talking sense, or day dreaming and missing a big inherent flaw?