And: I presume your service requires javascript to function (as all of the similar services do)... any chance of providing a non-javascript fallback such as a generic-looking checkout page hosted on your own servers (like when you use a paypal button to send users to their site to complete the final step of the transaction)? Please don't tell me that "everyone uses javascript yay" or that I shouldn't be concerned -- I know who my clients and their users are and I know what their accessibility requirements are :) (I always have to say this when I ask eCommerce services this question because usually they dismiss it out of hand and it's very frustrating when you have requirements that sites work even without javascript).
Thanks, and best of luck!