advice - repair shop did not replace part

[Anonymous]

I tried a new repair shop that was well-recommended on my neighborhood email listserv.

Repair seemed to be good, until I got on the highway a few days later and heard a grating sound -- looked under car and could see that mechanic had taken off entire front engine splash shield to do repair but then did not replace it. (I think I'm calling the piece by the right name - the front engine splash shield is a large plastic piece that covers the underside of the front engine compartment and protects it from the road mud, gravel, etc.) The grating sound was caused by a separate smaller shield in front of the front tire, which, now that it isn't connected to the engine shield, flops down on the road at high speed. I re-attached the piece by twist-tie, which allows me to drive, but it's obviously not an appropriate fix.

Originally, I dropped the car at the shop and asked for an estimate which was conveyed to me orally, which I then approved. At no time did the shop tell me that they would take off the engine shield but not replace it.

Now what to do? Replacing the engine shield wasn't part of the estimate. I feel like the shop should replace the piece for free for me - both parts and labor. Online, I can see that the piece is about $50. It's not possible to tell what the hourly labor rate is from the receipt I got back. Is asking for the engine shield to be replaced gratis reasonable? If not, what should I be asking for?

I feel like they're going to tell me either 1) it's not a necessary part (but isn't that my choice to make?) or 2) something popped up during repair that made them realize they couldn't put it back on (but then shouldn't they have told me that before they took it off? or consulted me during/after the process?)

Advice on how to handle?