A failed part does not always leave the shop with the car. Often the removed part is the only physical proof that a repair happened, and it is the easiest thing to lose at the counter. Before you hand over the keys, decide that you want it back, and say so in words the service writer will type into the file.
Ask before the work starts
Put the request on the work order itself, not only in a conversation at the desk. Ask the shop to hold any parts it removes and to keep them for you. Shops handle this differently. Some send removed parts to a supplier for a core credit or a warranty review, and some discard them on a fixed schedule. Ask which practice applies to your visit, who you should call if the part is gone when you come back, and whether the shop will hold it long enough for a provider to review a claim.
Tag the part so it can be identified
An untagged part is a part anyone can say was never yours. Ask the shop to write your name, the vehicle identification number, the odometer reading, and the work order number on a tag attached to the part or the bag holding it. Take a photo of the part in the bag or on the bench, with the odometer reading visible in the frame if you can manage it. Your photo is cheap, and it is the one record the shop does not control.
Read the closing invoice line by line
Compare what you approved with what you were billed. Check each part description, the labor hours against the job described, and any supplies, disposal, or shop fee lines. Confirm that the repair matches the complaint you made when you dropped the car off. If a line says a part was replaced for a reason other than the failure you reported, ask the shop to explain it in writing, and keep the answer.
What to keep together
Keep the original work order, the closing invoice, the part tag, your photos, and a short note of who you spoke to and when they said it. Keep them in one folder, not in your inbox, your glovebox, and your memory. If a provider reviews a claim, its file will depend on the paper you can produce, and a missing part is hard to argue around. Your contract may say what evidence it needs and whether it asks for failed parts or their records. Read the claim section for that language before you need it.
What to ask the provider before you buy
The shop is not the provider, and the shop's bench is not the provider's file. Ask the provider whether it requires the failed part to be kept, for how long, and whether the shop or you is responsible for keeping it. Ask whether you can request the part at any time and whether the provider will tell you in writing if it wants the part and cannot get it. A sample contract can answer several of these questions without a sales call, which is why this site lists the providers that post one; see providers that post a sample contract you can read first.
If the paper says nothing about evidence, do not assume the provider will be flexible later. Assume that the shop's records and your folder are what you will be arguing from, and set up both as if you will need them.