I'm charged selling fees for sales tax that I don't keep?

[librarypaced]

Sales tax is not payment to the seller.  Why is it included in the calculation of selling fees?

[disneyshopper]

FVF is based on item price + shipping cost + sales tax + .30

The FVF is 12.55% of item price + shipping + sales tax + .30

Before MP, the FVF was 10% of item price + shipping

Paypal charged 2.9% of item price + shipping + sales tax + .30

Total charges 12.90%