I've been working on a Flex UI for accepting payments utilizing PayPal's "Website Payments Pro (USA Only)." It's a little cumbersome getting going, but with patience and an insider's experience, you can be accepting payments with PayPal doing the heavy lifting for credit card validation. PayPal remains invisible, so you control the customer experience with your Flex UI.
I have a quick example that is using a test account - what PayPal refers to as a Sandbox account (duplicates the same processing as the Live site, but does not do any actual credit card charges). To try it out with a successful simulated payment, use this Visa number: ; expiration date: 1/2007; CVV2: 000. To simulate a failure, change either the expiration date: 1/2006 or the CVV2: 1234
* NOTE: Part of the process in accepting payments is running SSL. On my site, you will get a SSL warning because the certificate I generated has a mismatch in the name on it (M Givens) and the name of the site (webmxml.no-ip.info). Feel free to trust my certificate's credentials - I'm here to help.