TIE ROD END

You buy what needs to be replaced. Cars, and most light trucks, have four, an inner tie rod end and an outer tie rod end on each side. They form the steering linkage to each front wheel. If one is worn enough to separate, that wheel will turn wherever it wants to go, and will send you into the ditch or into oncoming traffic.

Where the inner and outer tie rod are screwed together is the final alignment adjustment. An alignment is required when one or more tie rod ends are replaced.

Unless you are planning on replacing the parts yourself, or the shop asked you to find rare parts, it is not customary to buy parts then expect your mechanic to install them. That is like bringing your own food to a restaurant and asking them to cook it for you. When the shop orders the parts, they assume the responsibility if one is defective, wrong, or they break it. When you supply the parts, you take on those responsibilities. If a new part fails while under warranty, you can be expected to take care of getting it replaced, and to pay for the labor a second time.