My customer is trying to pay an invoice I sent but QB will not accept the Canadian billing address of his credit card.

Thanks for coming here, Chey.

Let me share insights about paying invoices to QuickBooks United States from a credit card that has a Canadian billing address.

Intuit's payment processing only allows 5-digit numeric zip codes as standard in the US. 

I'd suggest you inform your customers to contact their issuing bank to determine if they have the option to use their international card with U.S.-based businesses that don't meet the U.S. 5-digit standard.  

If in case the international credit card can't be used, I'd suggest you contact our QuickBooks Payments team. They can advise you about other forms of payment so your customer can pay their invoices successfully. 

For future reference, read through our help article: Turn on and use multicurrency. It helps you learn how to set up multicurrency in case you need to record transactions using a currency other than your local currency. 

Feel free to message me again if you have anything in mind about QuickBooks. We're always here to help.