How important is to to have a hitch receiver cover if there isn't anything attached? A friend is helping me remove my hitch rack for the season tomorrow evening. I've looked online for the covers and while I can find basic ones for $3.75, it seems like all of the sites want $6-$9 extra to ship it. I am not paying $12-$13 for something that costs under $4!

I imagine that having it does help prevent rusting...ideas on local stores where I can find such a thing in stock? So far I've not had much luck...but I am probably just not looking in the right place.