Thanks for reaching out here. QuickBooks Online Payments is a great time saver that allows your customers to pay directly from their invoice. I'll be happy to provide more information with regards to multicurrency, so you're on the right track with your work!
QuickBooks Online Payments currently accepts Canadian payment methods only. If you have your customer set up with a default currency, there won't be an option for them to 'pay now' when they receive their invoice. They'll need to pay you using another method. In addition, when you create the invoice, the exchange rate will be applied and both currencies will be listed so your payment can be received in the Canadian dollar amount. I suggest adding a memo to advise the customer of your preference.
If you have any other questions, feel free to reach back out. We'd be glad to help!