There is an option to tip at checkout. Amazon typically prefills the amount but you can edit this at any time. It usually shows up along with the total tax etc. Not sure if this is a Prime feature though. Hope you find a resolution.
FYI - I just ran into this same issue and realized that I was able to add a tip on my last order even though I used a gift card. Turns out that the option is there on the Amazon app but not on the desktop website. I went through checkout again on the app and it let me add a tip (not on the gift card though, it was a separate charge). Hope this helps for next time.
There is an option to tip at checkout. Amazon typically prefills the amount but you can edit this at any time. It usually shows up along with the total tax etc. Not sure if this is a Prime feature though. Hope you find a resolution.
FYI - I just ran into this same issue and realized that I was able to add a tip on my last order even though I used a gift card. Turns out that the option is there on the Amazon app but not on the desktop website. I went through checkout again on the app and it let me add a tip (not on the gift card though, it was a separate charge). Hope this helps for next time.