Hello Brian,
Welcome to the QuickBooks Community. I'm happy that you've reached out here for support and that you've been using QuickBooks Payments to successfully receive payments from your Canadian customers. Let me explain why the option is greyed out for your American customer.
While QuickBooks Payments allows you to accept payment from your Canadian customers by credit card, you'll be unable to accept credit card payment from international customers in their home currency. This information is laid out in the Intuit Payments Acceptable Use Policy which I've linked below along with the Merchant Agreement.
When receiving a payment from your American customer, you'll be able to specify the payment method as Credit Card from the Receive payment screen, or select another method if applicable. Here's more information on how to Record invoice payments in QuickBooks Online.
If you have any follow up questions or need additional support please don't hesitate to let me know. I'm here to help!
