If you use a debit card to make Amazon purchases, those purchases may be treated as a PIN-less Debit. As I described last year
, PIN-less debit card purchases may be treated like a PIN-based transaction which may not be considered as a qualified debit card purchase by some reward checking accounts.
Amazon has a page describing
how to opt out of PIN-less debit card transactions. Credit for noting this issue and for the Amazon link goes to this FW thread