If you have to do your own diagnosis, and order the parts yourself, you might as well put on the new gasket as well, pay yourself the $400 for being cleaver, and tell Sears where they can cram it.
If you don't feel cleaver there are books at 1/2 price to help you put on the gasket. Come to think of it, there's probably a book to tell you how to tell Sears to cram it too.