Some car owners buy the part themselves and ask the shop to install it. Others leave the sourcing to the shop. The choice matters for a service contract because the person who selected and supplied the part may be the person that the contract, the shop, and the part maker each point to when something fails. Before you choose, understand how that split of responsibility works.
Who stands behind the part
A shop that installs a part it bought usually stands behind that part and its own labor under its own terms. A part you bought yourself may carry a manufacturer's warranty, but that warranty runs from the manufacturer or the seller, not from the shop. If the shop installs a part you supplied and it fails, the shop may say the failure is not its responsibility, and the part maker may ask for proof of purchase and a failure report. Ask the shop how it handles customer-supplied parts before the job starts.
What your contract says about outside parts
Read the contract for language about aftermarket parts, customer-supplied parts, and repairs done with parts that did not come through an approved channel. Some contracts exclude failures caused by a part that is not approved, and some require that replacement parts meet a stated standard. Others say nothing. Where the contract is silent, ask the company in writing how it treats a claim involving a part you supplied. Get that answer before the work is done.
Questions to ask before you decide
Ask the shop whether it will install a customer-supplied part and whether it will stand behind its labor if that part fails. Ask the part seller what warranty it offers and whether that warranty covers labor at all. Ask the contract company whether a customer-supplied part affects any claim. Keep the receipts and written answers together in one folder, along with the part packaging and any installation instructions.
When handing it to the shop makes sense
If the part is specialized, if the shop has a record of installing it, or if the contract requires certain approvals, letting the shop source the part often keeps the chain of responsibility simple. If you are comfortable with the work and the contract is clear about what it expects, doing the sourcing yourself can work. The contract and the shop's written answers should decide the matter, not the appeal of a cheaper part on a website.