I would suggest sending a complaint to Delta after your trip finishes. Keep it simple and express concern that the agent required luggage wrapping on a bag that was properly packed, and thus not following DL’s own published policies. Include the station’s identifier as well as either the agent’s name (if you remember) or initials (these are located on the bag tag under “agent” such as ATL/MK whereas MK would be the initials. You’ll likely get some sympathy miles, but the comment should be forwarded through to a team somewhere. You never know if that particular agent is getting a lot of complaints, so it can’t hurt as a datapoint. Who knows, you might get miles financially equivalent to more than the cost of wrapping.