Code seems to work once per account when you pay through Paypal and register payment method. Not sure if it works with other payment methods.
If you want to un-authorise Gray after your transaction is complete:
1) Login to Paypal
2) Under recent activity choose "Details" of your Gray's purchase
3) Ensure the status is complete
4) Go to "View Billing Agreement Details"
5) "Cancel" future authorisation
Just used it, thanks. Interesting that when it gets to the Paypal checkout it doesn't let you specify the funding source like most other transactions through Paypal. It just uses "default funding source".