Hi there, info1290.
To add multi-currency in the invoice, you'll need first to turn on this feature from the Account and Settings page.
You can check this article for additional details: Turn on and use multicurrency. This also provides steps on how you can delete currencies.
Once multicurrency is turned on, I suggest going to the Customer's information page. From there, you can assign a foreign currency to your customers to appear in the invoice. Please follow the steps below.
For additional information, you can click this article: Assign a foreign currency to a customer in QuickBooks Online.
On the other hand, we're unable to call you directly from one of our support. However, I recommend going to the Help icon. From there, you can contact us through messaging so that our QuickBooks Support Team can discuss the concerns with you. Please note that we also have limited staff and reduced support hours due to the pandemic. They're only open from 9:30 AM to 6:00 PM Monday-Friday.
Please refer to this article to see different information about before and after turning Multicurrency on in QuickBooks: How Multicurrency works.
If there’s anything else I can help find in QBO, let me know by posting a comment below. I’m more than happy to assist you further. Enjoy the rest of the day.