[Fat Charlie]

Aftermarket parts can be great, if you're confident in the listing and doing the work yourself. Most shops won't be willing to install customer supplied parts for a long list of perfectly good reasons- and unless that warranty pays their labor (at their labor rate) then you're going to be paying for that second install. If you're paying someone to work on it you're best off having them handle the parts.

I'm a parts guy at a dealership and we've got no problem with most aftermarket parts- if I'm happy with the listing and the tech doesn't have a bad feeling about it. The tech isn't going to waste his time on a job like yours that might have problems because he's on his own. I can't support him and the shops I buy from can't support him. You are his parts department for the job- "Call customer Smith and tell her we need a gasket for that kit she bought on the internet." Once he's gotten through all that he knows he's going to face the "ever since you did X job I've had Y problem with my car!" It's not something most places would touch because it's not worth it.