1. Shops don't stock parts.
2. Shops don't order parts over the internet--with the rare exception of special projects.
3. Shops will source the parts through a local parts store, and the shipping/handling/parts store profit has been added to what he pays, then he is going to mark it up 35%. Now, I am willing to bet that he isn't even marking it up the full amount for you.
I have people all the time tell me they can buy the parts online cheaper. I tell them that is fine, and if they want me to install the parts I will be happy to do it UNLESS the parts are crap, then I won't install them, I also inform them that I won't provide ANY labor warranty on parts not sourced through my shop, I can't fight with some online shop for an adjustment when their part failed on a customer, and the nail in the coffin--for those vehicles that are not operational--is the storage fee I will charge while their parts are getting shipped.