I use [0] on Chrome. [1] exists for FireFox, too.

[0] https://chrome.google.com/webstore/detail/smile-always/jgpmh...

[1] https://addons.mozilla.org/en-us/firefox/addon/amazonsmilere...

Since I already use it for other things, I use https://chrome.google.com/webstore/detail/requestly/mdnleldc...

Just have a rule that changes www.amazon.com to smile.amazon.com and it works for me.

Only because you ask, my company at https://givingassistant.org/coupon-codes/amazon.com has a deal with Amazon that donates 1.5% on top of their 0.5% to make it a full 2%, with no category restrictions. Start shopping here and we direct you to smile as a starting point. Free, and if I don't have your charity on file, ask and I'll add it.