The hard part isn't doing credit card payment. It won't take you fifteen minutes to integrate, e.g., single purchases of Paypal content into a typical Rails app. Trust me.
The hard part is you just introduced requirements for users, authentication, logging, administration, bookkeeping, accounting, and reporting into your app. And those requirements are, necessarily, different for everybody.
Its the same reason Rails doesn't have a One Way To Do It user signup & administration feature: everyone's needs are different and the overhead added by standardization is greater than the engineering effort saved by it.
(Sidenote: if you don't need subscriptions www.e-junkie.com and don't look back. $5 a month, saves a lot of boring report & email integration when you're starting. It took me about ten minutes to set up "People click this button to pay me, e-junkie mails them a license key." when I started. I didn't add anything to their solution for about 2 years. These days they call a script on my server so I can do stat tracking to my own satisfaction, but the core loop hasn't changed much at all.)