I agree strongly that Amazon needs to make this clearer in their emails an admin pages (listing earnings that don't exist is
terrible) but as far as their agreement goes, I think you are mistaken.
Section 2.a. of the Amazon Appstore agreement, emphasis mine:
"a. Royalty. For each sale of an App, we will pay you a royalty (“Royalty”) equal to the greater of (i) 70% of the purchase price or (ii) 20% of the List Price (defined in and subject to section 5i) as of the purchase date. No Royalty is payable for Apps with a List Price of $0.00. Such purchase price excludes taxes and any separately stated fees or charges. A Royalty is due only for sales for which we have received final payment from or on behalf of an end user. If an App is purchased using a credit card or bank account deduction mechanism, final payment will be deemed to have occurred when the applicable credit card company or bank has fully settled the payment for the applicable purchase."