You really need business advice in addition to any technical advice that we can offer. Try talking to your bank about their merchant services and find out what they can offer in terms of online payment processing. There are a couple of reasons for this. Firstly, they are responsible for the security of the transaction and as such will have built and tested a reasonably secure system. Secondly, as a new business, you may not be able to get a credit card merchant account until you can show that you are a legit business with a number of years credit history.

good luck.