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The Tax rate for your store is set visible here. Your store will be set to the default country value but you are able to edit the value hereand is not editable by the store admin, this will change automatically should the defualt contry Tax value change.
You can use this tool to add additional tax rates for additional countries.
Setting a new tax rate
Navigate to the tax section by selecting Payment Options>Taxes
By pressing the "create" button you will be presented with the below screen:
Here you can give your new tax a name (e.g. VAT), set a code to be able to identify it and set the percentage at you wish the tax to be.
It is possible to set up multiple tax rates from this section.