When you figure actual shipping costs, the time and effort to pull a small part from stock or off the shelf, package it, get it to the shipper (small businesses may not have pickup), and mail it - $6-$7 really isn't bad. Unless the gun store is really big, you can't expect to find parts for every make and model gun on the shelf. The store will stock what sells. If they get several requests for a part, they will stock maybe one or two. The first two customers who see it, get it.
It's a bummer, for sure, but like my Daddy used to say, "It may be crooked, but it's the only game in town."