Surprisingly, no, you can't - it skips over your gift card balance and pulls from your credit card. I think Kindle Unlimited subscription might be the same way. Actually buying kindle books looks like it will charge your credit card but actually pulls from your gift card balance instead like it is supposed to.

Staples rewards are only 5% once you pass a $$ threshold now, so I don't know if the first $XX are actually at 5% anymore, they might only be 2% and 3%.