It's not really a discount, it is a fee they impose. In the item list create a discount item called amazon fee, not taxable, and linked to the expense account you wish.
use that item on the sales receipt and enter the amount of the fee.
the sales tax issue requires a work around, open a sales receipt
line one - a service item for the total amount of the three invoices, set to taxable
line two - the same service item for same amount, but set the qty to negative one, and set it to non tax
the invoice will total to just the amount of the tax
in receive payments, select that tax invoice, and in cash back select an expense account for sales tax paid, and enter the full amount, save