(I just got my car back from the shop a week ago under similar circumstances, I had shop A lined up to do the work, eSurance didn't allow shop A to do estimates so I took it to shop B. Shop B didn't take off the bumper and assumed it could be salvaged and repainted along with the rear diffuser, so they quoted $1040. Took car to shop A who was highly recommended and I've seen their quality work, they took the bumper off and said nope, we need $900 more because this bumper has broken tabs and the diffuser is fucked, so they dealt with eSurance to prove it and get the money, I didn't have to do a thing... then 2 weeks later I pick up the car and sign over the $1040 check I got from eSurance and drove my car home.)
I'm not sure they would've been ok with my enterprise rental car another week for waiting on aftermarket rear bumper... I didn't want to deal with it so I got 100% oem