This might appear obvious to some. But I wanted to be sure.
The basic flow here (if using the checkout_button) is :
On your site create an order
Get to the "checkout process" where we generate a page summarizing the order with items put in a Cart and a checkout_button for that cart is created.
Clicking the button takes you to google. When your order is complete at google. The user goes off to do whatever and your site should implement a notification controller which handles the updates from Google letting you know when the final "authorization" is approved for the fee.
or am I missing something? Is it even simpler than that?