title: Variable amount recurring billing
body: <p>Hi i'm looking for a payment gateway that can do recurring billing that changes month to month.  Fogbugz do this, they charge based on how many active users there were that month. All of the APIs that i've found only let you set a fixed amount and it's difficult/impossible to vary the subscription amount month to month.</p>
  
  <p>Has anyone come across any services that do this?</p>