I have this one, but it's for the domain pricing list, and it doesn't show up when a domain is available and ready to register.
I believe this is the relevant bit:
This is what I'm talking about, where the 'more info' drop down menu appears when a domain is found to be available. It provides the registrant with a choice of registration periods, but no option to adjust the currency if different currencies are being used. Given that the registrant has not necessarily registered as a client yet, the default currency has not yet been set for them (which is done later in the cart), so it looks like a missed step in the program logic.
BTW the support developers told me: "Clients can change their currency on the checkout page, but not the domain results page of the order form I'm afraid. However we welcome suggestions for new features on our feature request system: http://requests.whmcs.com." I can't wait around for it to be implemented, as it is an essential step. Any suggestions?