Recurring billing with Rails and ActiveMerchant: Best practices, pitfalls, gotchas?
One thing I wanted to add: keep in mind you don't need to use the recurring billing feature that is built into the gateway. In general these systems are legacy and very difficult to deal with, we get spoiled in the rails world.
You get a lot more flexibility just using them for one purpose (to bill a credit card, and perhaps also store credit cards for PCI compliance). Then roll your own recurring billing in your rails app with a cron job, a date field for when they are paid through, and amount each person is paying (in case they used a coupon) etc.
One small example: sometimes people will cancel a monthly subscription in the middle of the month. They want to make sure they don't forget to cancel before the next payment. Most gateway recurring billing that I've seen will instantly terminate the account (or send you a message indicating this). In reality, the user has paid through the end of the month and should be given 2 more weeks of access. You can do this if you have rolled your own recurring billing in rails, but not if you are using the gateway recurring billing. Just a small example.
RailsKits has a Software as a Service kit that should do what you need. It has built-in support for free trials, upgrading, downgrading, plan limits, etc., and it supports PaymentExpress (and some others).
I've researched it a bit for a project I'm doing, but I haven't purchased it yet so I can't vouch for it. However, I have seen a few blog posts praising this kit.
While the RailsKit is relatively inexpensive when compared what it would cost you to implement all of its features yourself, there are a couple open source versions out there that aim to accomplish the same thing. The one I remember off the top of my head is called Freemium.
EDIT: I forgot to mention that Ryan Bates said in his most recent Railscast that his next episode or two will deal with recurring billing, so keep an eye out for that. He usually does one episode per week, and the five he's done since December 22 all cover handling payments of different types.
Peepcode has a PDF for sale(70 pages) that details various aspects of payment processing and industry practices for this. It may be worth checking out: