Used this 2x today. I noticed that for some of the amazon deals it applies to the pre-discount price, and for others, to the post-discount price. For example, the Fire TV Stick.
The regular price is $39.99, the Prime Day Price is $19.99. When I applied the Amex point promo, it took off $8 (20% of 40) and not the expected $4, so now it was $12+tax!