It will show the currency options that you select in the multi currency addon’s settings menu. The multi currency works for standard/normal one-time payments also.
The issue you explained could be related to page caching. I have explained the caching issue below so you can check your site and server setup to see if there is any caching happening on the page containing the payment button/product:
The e-commerce functionality needs a dynamic page being shown to the visitors (so they are seeing the latest values/data of a product). A cached page simply shows a static cached page (our plugin doesn’t get a chance to apply the latest calculation and price to that page). When a cached page is being shown, the user is not actually being shown the updated price values of a product. That’s what causes the issue with e-commerce type setup because an e-commerce type site is not the same as a blog type site. Caching is normally fine with a Blog type site where there is no need to show updated content.
Exclude the pages (containing the payment buttons) from caching. Then try again to see if that helps.