Hi Samuel:
In the absence of If/Else logic, three approaches come to mind (you may have already considered these):
1) Have 2 versions of the invoice spec file, one for USD and one for CAD, then assign the USD spec to the USD Customers.
2) Print an Optional field on the spec file and enter "USD Dollars" on the Customer profile but leave it blank for CAD Customers.
3) Since the Customers are likely in 2 different Control Account Sets, make the Description of the USD Account Set = "USD Dollars" and print it on the invoice (spec code = E052). The Description of the CAD Account Set would either have to be blank or say "CAD Dollars".
The third option only requires one spec file and probably comes as close as you can get to the simplicity of the If/Else option.