Level 2

Sales Tax

We just started selling thru Amazon and I have a couple of questions.  1. I did not include sales tax on 3 previous invoices.  How do I put that into quickbooks so that I pay it even if we did not collect it.  2.  Amazon is taking a discount with each sale.  Currently I am putting that as an expense on the invoice, would that be taxable or non taxable and how would I get the sales tax amount to calculate accurate in that instance?