Fwiw, see my diy thread on this install. Pricing for parts listed above isn't accurate. As was my experience, many service centers won't install seats you bring in.
When my SC wouldn't do it, I discovered you can get the rear reinforcement bar installed by an authorized body shop. Not expensive. I paid $315 for the part, labor and tax.
The other small parts you need you can buy directly from tesla - T-slot nuts, screws, left and right latches, tailgate switch. About $225 if you need all that.