Please take a look at the following link [gizmodo.com]
and some quotes from its context.
"...After finishing the pre-order, however, his credit card was listed as paying the full amount on the receipt. The problem, he told Gizmodo via email, was that Google said it would only charge him once the product shipped, and Stadia was expected to ship after his store credit expired—meaning he'd have to pay full price for Stadia.
Others shared similar stories on Stadia's subreddit. Multiple users found their purchase going through, only to discover that their card was listed as paying the full amount. At least one person claimed Google canceled their pre-order, and another noted they couldn't even attempt to use their store credit on a pre-order.
After reaching out to Google about the problem, we have some good news. If your store credit expired before July 1st, and you tried to use your credit to pre-order Stadia, you can contact to Google's customer service to get things fixed. If your store credit was not set to expire by July 1, it should now be extended to January 1, 2020, and it will cover your Stadia purchase when the service launches. In any event, go ahead and contact Google support. We've been told they're ready to help."
Good luck and cheers,