Foolishly put a baby seat (without baby in it) in the trunk to make room for adult passengers and though it shut, a few days later I noticed a little dimple in the outer surface of my trunk lid. Must have been part of the seat which pressed up underneath the lid.

What to do? Any lost cost alternatives to smoothing it out?

if its dented? try finding a paintless dent removal place and see what they say. otherwise body shop or put up with it