Just want to add my experience since there's barely any post on Hawaiian Airlines!
Amex PRG
Hawaiian Airlines 50$ gift card
Posted as "Hawaiian Gifts"
Classified as " Merchandise & Supplies - Internet Purchase"
12/08 Posted
12/10 Reimbursed
And if this helps anyone, I purchased United registry,then called Amex to change my airline to HA before getting reimbursed for United since I thought it doesn't seem to work and I prefer Hawaiian. Ended up getting reimbursed for the United registry too.