I've always been successful with adding different coupon codes at the counter. I usually add one United coupon for 500 miles (MUAA084), which cannot be added at booking time bcs of travel agent. Mostly I do it online by editing my rental at MyAvis, but cannot always. I've also previously added the free weekend certs either at the counter when renting or dropping of the car.

Yesterday a Avis representative at JFK told me it was against their policies to add electronic coupon codes at rental time and told me "this time you have to bite the dust". Very nice and that was in front of my wife and kids.

Anyhow, is this really the Avis policy not to add coupon codes?