Any shop that tells you to drive a car with a leaking rear main seal knows full well they screwed up. Having worked on Pontiacs over 50 years, no rear main seal ever stops leaking from driving it. There are no short cuts at this point, engine needs to come out, put on an engine stand, and do the rear main seal over.
As has been said, it's a common problem, always has been. Pontiac isn't the only engine that used the rope seal 2 piece design, and they all have to be done 100% correctly, or they leak.
Make them fix it before you pay for it. No, you should not have to pay for anything over, and above what you current bill is. There is an inferred parts, and labor warranty on repairs, and they're on the hook for it. The car hasn't even left the garage, and there are problems.
I have no idea what they used for a rear main, but BOP makes 2 different styles of synthetic replacements, a one piece, and a 2 piece design. You might suggest they try one of these if they aren't sure how to properly do a rope seal.
Link to BOP seals:
https://www.bopengineering.com/A_Pon..._Seals_1.shtml