I must add that there is one more variable here that you may not have considered and it may not be true in your case.
The dealership is probably selling you GM OEM parts, and the parts store is selling you aftermarket. Like I said that may not be the situation in this case, but possibly.
Just remember that an OEM replacement at a GM garage brings with it the warranty on parts AND labor, most aftermarket parts warranties only replace the part. (There is a cost to pay for that peace of mind).
Thank you for letting me share my