I am looking into setting up a payment processing solution for a site I've developed. The site is for an annual convention and my client wants to handle pre-registration (MC, Visa, and AmEx)starting in August and running up until December when the convention is held. We have decided that we will purchase a secure certificate so the processing can be handled on our site. My question is which payment processor would work best for us? Since we won't be using the processing from January until August it doesn't make sense to have a high monthly fee for the processor. Any thoughts would be greatly appreciated.