I think I have your answer, given that we are NOT talking about the card verification process here.
If you live outside the United Kingdom or the Eurozone, your BANK will charge you an international currency transaction fee on top of whatever the subscription cost ends up being in your currency. This fee will vary depending on your bank and local financial regulations. Here are some examples, for a single character account in each case and assuming the bank's charge is always 50c (inevitably not representative of your circumstances).
Example #1: United States
Charge of $9.99US per month from SE charged through ClickandBuy
Charge of $0.50US per transaction from your bank for paying a UK/EU company in their currency.
Total: $10.49US
Example #2: France
Charge of 9.99EU per month from SE charged through ClickandBuy
Total: 9.99EU
Example #3: Australia
Charge of 9.99EU per month from SE charged through ClickandBuy
This converts to $12.64AU
Charge of $0.50AU per transaction from your bank for paying an EU company in their currency.
Total: $13.14AU
So as you can see, you will need to watch what currency you are paying in (any worldwide) and what that currency is being converted to (always Euro or pounds with C&B) and if there is a conversion, your bank will charge you an extra fee.
Now SE have had complaints about these fees on the FFXI forums and have promised to "fix" it. I want to point out that these fees are entirely beyond the control of SE or ClickandBuy. In order to fix it, they would have to reduce the service fee costs they charge to "mask" what your bank charges you and this would of course be open to exploitation, which means that won't happen. Alternatively, it would require fundamental changes in worldwide monetary policy - of the kind not happening any time soon.
Lastly, C&B charge an annual admin fee if you load your account with cash, but this should not apply in your case if you are going through the SE management system - only if you are using the C&B system for things other than this game.
((PS: I still want that alternative billing option that was promised...))