Using a 3rd party credit card processing gateway obviates the need for storing credit information on the client's server - the POST'ed cc info is passed along to the processing gateway which returns a transaction id that can be used for record-keeping by your client.
A credit card payment gateway is provided by companies like Authorize.net, LinkPoint Central - even PayPal is getting into the game. All the major gateways have existing code for integrating a shopping cart with most of the popular web programming platforms (.NET, PHP, Java, etc.). Plus most major shopping carts support the major gateways out of the box, or at the very least have installable modules for most gateways.
So, your client should get an internet payment gateway setup and you should integrate their existing code with the gateway.
