This is a great question and I'm happy to go over this with you.
When it comes to QuickBooks Payments, the program accepts Canadian currency transactions. Customers set up to pay in Canadian currency in QuickBooks Online will have the ability to pay using their credit cards. Customers set up for other currencies, such as USD, won't have the button to Pay Now on their invoice and enter their credit card.
For customers that are using currency other than CAD, I recommend looking at third party apps or other options to give them the ability to pay online. You can take a look at what connects with QuickBooks by going to the Apps tab in your account and using keywords such as "payments" to search.
For your reference, here are a few articles about QuickBooks Payments.