I think this will be a little trickier that it looks. If you would like to offer the user a choice of currency to pay for the item you would need to be able to show them the total for their purchase in the other currency. You would then need to update you checkout code for the currency to use the value that is set from the user's selection. Lastly you would need to ensure that your merchant gateway will allow you to accept foreign currency.
Assuming that the gateway will allow it updating the checkout server behaviors to use your new currency should be easy enough. The hard part will be converting the values to the other currency. This is a problem because of the variance in the conversion values of other currencies. There are some services out there that can take a value in one currency and convert it to another but this is not a feature of any of the WebAssist tools and would be custom.
There are some other aspects of this as well, such as the order summary that is stored for the transaction. Do you store the native currency value or the paid currency value or both?
These are the biggest problems that I can see and these parts would take some investigating and planning as well as custom coding to implement.
