I can help you with adding customers in CAD, @bcpatters.

The Multicurrency feature is only available in QuickBooks Online Essentials, Plus, and Advanced. From here, you can turn on this feature from the Account and settings. Once done, create a profile for this customer and set their Currency as CAD Canadian Dollar.

You can follow the steps outlined by Mark_R above on how to turn on the Multicurrency feature and add the CAD Canadian Dollar as your Currency. Check this article for more information: How to add a foreign currency account. 

From here, you can now create invoices in CAD. Refer to this article on how to create invoice and record payments: Create invoices in QuickBooks Online. 

Let me know if you need further help adding your customers in CAD. I'm always here to help. Have a great rest of the day!