What's the most simple way to accept monthly subscription fees?
lunaru: Most of the responses here have been in response to whether or not there are apps out there that do this for you.However, if you want to go down the road of building this yourself, it's not really that involved. Take a look at ActiveMerchant if you're doing Rails. It helps you integrate with a good list of payment gateways. Then, sign up for that gateway + get a merchant account. This is probably the best way to get the lowest fees.For example, I believe CDGCommerce offers merchant accounts at $10/month with a free payment gateway. The typical $0.30 + 2.9% (or whatever your rate is, depending on risk, etc.) applies.Another path for this is Website Payments Pro via Paypal. With IPN you can easily sync your user subscription status with the latest payment information.Doing recurring billing via subscription fees, like all programming is handling corner cases. For this domain, the corner case is a combination of invalid credit card numbers, expired numbers, perpetual zero-balance but valid numbers, overzealous anti-fraud filters, overdrawn accounts, or even temporary credit card numbers. Which ever service or integration path you choose, make sure you're covering the these corner cases.Based on all this, my advice is to build in-house so you have the flexibility of changing policies on-the-fly with regards to how you handle those above cases. YMMV.