Your mistake was the desire to get back on the road ASAP with two brand new tires rather than call Avis the moment you reached Cottonwood. There was probably a phone or cell reception in Cottonwood. You also need to figure out if there is a nail in the flat tire because that is on you if that's the cause of the flat. There are various ways to handle this situation, but buying two new tires for Avis or Hertz would never enter my mind.

For my own cars, they always get new tires. But a rental car is not my car.

If it turns out I hit a nail, they're getting a used tire from the used tire shop or I'm having the used tire shop plug the hole. I'm not telling the rental agency they got a different tire or that I plugged a hole. I'm supposed to tell them but that ain't happening. Used tires are not a good idea, but they're good enough to get back to LA. I'm not paying full freight brand new for a tire I'm only going to use for the 1st 300 miles of it's 30000 mile life that benefits Avis instead of me.