You can add which currency you are using by editing the HTML in the output pages:
You can do this by going to settings, e-commerce, output pages. Select which output page you would like to edit (i.e. "Invoice").
Next scroll down to the HTML and simply add "USD" to the order total. Lastly be sure to select "Save changes" on the right navigation bar.
The example above shows the editable HTML box that is found on the output page that you are editing. The arrow points to the "USD" that has been added to the HTML and will now show up on invoices.
