Using the tax feature
This option can be used to charge a percentage extra on the basket total at different rates by the players country/state. You can find the setup page in the "Settings > Taxes" section of the control panel. On this page, you will be able to view and edit your existing taxes and create new tax rules.
When you create a tax rule you will be able to choose the name and the percentage. The percentage field determines the percentage extra you would like players to pay. The percentage set will then be added to all purchases from the specified "Country". You can also narrow this down to the specific state/provinces as well using the "State" option.