Normally, you buy the upgrade and they send you the files via e-mail - you just replace the files on your card, using any card reader connected to your PC.
Just send an e-mail to:
[email protected] (AvMap Orders) and they usually reply quite quickly.
Nowadays, you can also use credit card to pay (until very recently it was the case of bank transfer in €, with all the usual b...y bank charges)
But all the above applies to "upgrade/update" ... I am not sure about switching from US to Europe?