Credit card rebates for friend's purchase?
I am trying to figure what's ethical in this situation. A less-well-off friend asks me to buy things on Amazon so he gets the fast Prime shipping I pay for annually. He pays me back promptly. After doing this for years, it dawned on me that I get a 5% rebate on those purchases, because I use my Prime credit card.
It seems awkward to start rebating him the 5% after not doing so for years. His typical purchase is about $100 every few months. I do this friend many other favors, including driving 90% of the time when we go somewhere together (I have never asked for gas money) and getting him a nice birthday gift each year.
Ethically, should I be refunding that 5%? If so, what's a way of doing it now without seeming that I took advantage in the past?
In favor of rebating: I am benefiting from doing him a favor, which is not my intention, and the money means more to him than to me. Also, since he doesn't know the purchase is benefiting me, it could be considered I am getting a commission on his purchases.
Against: The amount is not much in the scope of our friendship, and it could be thought of as letting him share the cost of the Prime membership.