When I switch to USD currency to charge client (my home currency is CAD), the online payment option turns off.

Afaik, QB Payment doesn't offer such feature. Consider having a 3rd party app or a 3rd party merchant service to accept international payments.  

Hi there Floodesignstoronto,

It's important that all your transactions be entered correctly so that your customers receive the right information. QuickBooks Payments is a great way for your customers to pay you directly from invoices you've sent them to help make your transactions easier to process. I can provide some information about payment processing.

As stated by Fiat Lux - ASIA, QuickBooks payments can only process payments for transactions that are set in CAD, so that would be why you wouldn't see the option for payments with USD invoices. One option to process payments for multicurrency transactions would be to use an app made by a third party. You can find a selection of apps able to make a connection with QuickBooks by going to the Apps tab. You can click on them to view info about the apps as well as customer reviews to help you make the choice of the best app to help you with your business.

If you have any other questions about QuickBooks, feel free to reach out here.