How to convert an order price to a different currency in Commerce 2?
This is what I've done for now.


Add the currencies you will use in the UI. /admin/commerce/config/currencies
Add a "currency" product attribute in the UI. /admin/commerce/product-attributes
On the product edit page for the product you want, add variations with the currencies and set the prices.


This lets the user choose the currency he/she likes. So you can just put a message "If you're using JCB, you must pay with JCB."  It's not perfect but it's a decent solution.