I use authorize.net for my billing. I think they're pretty standard. They have a recurring billing API. If you want to do the whole "store credit card" thing

I use authorize.net for my billing. I think they're pretty standard. They have a recurring billing API. If you want to do the whole "store credit card" thing, without storing CC info on your servers, look into their Customer Info Manager product. Their APIs are OK, mediocre documentation, Ok sample code. Not great, but they're a standard.