pcmillhouse,
it has nothing todo with your tax.
Your products costs 90 and your shipment 10. So you have to pay 100 euro, right? The payment has a fee of 2%, so if you have to pay for 100, you must pay with fee 102, right? But the costs for 102 are not anylonger just 2, the costs are still 2%, but of 102 now. => So the costs for the payment are now 2.04, alltogether 102.4.
It has nothing todo with the tax, only with the fee