I do know the "true bottom", but it is 200 pages of documents, and will depend on the mix of debit cards, Amex, and card levels such as Business/Signature/etc, and international, and average transaction amount.

If you are interested Visa alone, then the latest fee schedule is here:

http://usa.visa.com/download/merchants/Visa-Interchange-Reim...

1.45%-1.65% + $0.10 is a very common interchange for retail. The 1.75%+$0.00 pricing is going to be zero margin for them. If some merchant just uses Amazon for Amex, they will lose on that merchant. If the merchant primarily serves debit card customers, Amazon will make a small amount of money.

Large volume can get costs of infrastructure per transaction absurdly low; the true floor is the interchange rate that you pay to the card issuer; it depends on type of card but it can be ~1% for many of them. American express is much higher, taking amex at 1.75% is definitely a loss.

I'd assume so because the 1.75% is available for a limited-time only. The fine print mentions that the standard rates (after the promotional period) are 2.5% and 2.75%.