Originally posted by Lowell Goudge the solution is simple
failure to return originals is theft of copyright material.
You should notify the copyright office and cc. the store and corporate office.
that will sort it out in very short time.
Sadly, if it's stated anywhere that the negatives won't be returned, then dropping off the film would constitute acceptance of the policy, and therefore the claim of theft would not hold up. Don't get me wrong, I still think it's wrong, and foolish, and the epitome of corporate pennypinching, but theft it's not.
If I were the OP, I'd send my film off to a reputable lab like Dwayne's or North Coast Photographic Services. Those places need the patronage, and you'll get much better prints and negs back. Any lab that doesn't return negs doesn't not deserve a film photographers hard earned money.