I am building a site where people will sign up and pay to use an online web app (a yearly fee). What I've been looking for, but can't seem to find is a very simple checkout/order processing component for this type of scenario. I really don't want or need a full-blown shopping cart with product pages, etc., since we're really only selling one product. Just need to be able to have a simple checkout page to confirm they want the subscription and enter a coupon code (optional), and then provide a variety of payment options. Initially I only really need PayPal Web Payments Standard integration, but would like if it already has hooks for others like Authorize.NET in case we add that in future.
Is there something out there like this, or am I going to have to roll my own? I don't want to re-invent the wheel if I don't have to. I realize I could dig into one of the open source carts like dashCommerce or nopCommerce and try to pull out the appropriate pieces, as well. My only problem is that they are pretty much tied to a data model that revolves around typical product/shopping cart situations.
I look forward to any ideas you might have.