How to add currency conversion to Order Printer invoices?

[Ajevie]

How do I add this line from my Orders page to my printed invoice in Order Printer?  "Paid by customer $62.98 USD (1 USD = 0.88911 GBP) £56.00 GBP"  My customers can pay in their local currency, but I need invoices printed in USD.

There used to be a wealth of coding information available on here, but suddenly, I can't find much of anything.