Making it a pay service

Rails 3.1.3

Hi. I have a general question about making a web service
premium-version.

A rails service can ask the users to pay for premium services.
I need some concrete samples of deploying such functionality to the
rails app, if any, including necessary gems, (ActiveMerchant, maybe?)

The methods of payment may include PayPal and CreditCard.

Does anyone have suggestions?

soichi