The currency for manual payments is selected by checking the location of your login IP address. If you are using a VPN or are traveling the currency can be different from the one you are usually using.
You can see the current currency on the bottom of the page:
You can also click on the currency to select a different one.
The currency for automatic payments can be set in the account settings under My Account -> My Profile -> Autorenewal payment currency:
This currency will be used when a service is setup for automatic renewal.