I have been told that the coverage would only be up to the amount put on the card. Thus if you purchased a $500 camera with $350 in gift cards and $150 on Amex, and you filed a claim using the extended warranty (or any other protection for that matter), the maximum payout would be the $150. Note also that if the receipt includes tax, delivery, etc., that's excluded as well. Assume that they figure you paid that stuff with Amex further reducing the potential claim.
Note that this was something I was told, not at all sure this allowed based on the t&c, and haven't put it to the test. Note also that this is specifically not true for the car rental coverage (many, many discussions on that).