What is the TF charges?

Hello,
The number of sales is 3. And each of price $10.
But my main balance showing $8.28.
So what is TF fees?
please provide the link .

Fixed Buyer Fee’s on TF:

Gross Income = List Price - Buyer fee
Gross Income * Exclusive Author Fee (12.5%-37.5%) = X1
Gross Income * US Withholding Tax (0%-30%) = X2
Net Income = Gross Income - X1 - X2