I don't think you'll be allowed to take credit card numbers through the app (and as a customer of the app store, I'm not sure I'd want you to!) If you support in-app purchase, apple will handle all this for you, and just send you a payment at the end of the month, but they'll take 30% for the service.
Look carefully at what Lodsys are up to before going down this route anyway - they're suing developers who implement in-app purchase for patent infringement. Take a good look at that before deciding what to do/how to do it (and if you're doing this for them as a contract, check the contract to see who is liable if anyone gets sued for infringement!)