The way the currencies are displayed in WHMCS is not intuitive.
The display format ought to change from the current drop-down list to radio ( ).
If a customer clicks on an external page that links to the billing system, starts going through the check-out process, the other currencies should be visible and not hidden.
Potential customers tend to act like sharks, don’t have time for small talk and when a page layout forces them to break the flow, you simply lose them.
Most folks are more concerned about the cost of the product/service and may not even look at the Currency option drop-down.
If you are a WHMCS user and think that this idea merits attention, please upvote it so that the WHMCS Dev team can re-think this.