title: Can I store a PayPal account to be billed later without an authorization amount?
body: <p>My service's pricing is based on usage. There's no monthly/weekly subscription, it's one off charges for however much the service is used on that day.</p>
  
  <p>If a user authorizes my service to charge X, but uses the service such that his bill is more than 15% or $75 more than X, I want to be able to charge the user's account appropriately. </p>
  
  <p>PayPal seems to let me create an authorization for a set amount, but it won't process my payment if it's 15% or $75 more than the authorized amount. Is there an API that let's me authorize my service for charging the user's account without an amount?</p>