Pre-payment for EGD, using BCBSIL in CA

I recently had an upper endoscopy that my GI/liver doc wanted to perform because of recent abdominal pain and reflux and I have a history of liver disease. The hospital charged me $1300 before my procedure and I stupidly paid it: I assumed this was what I owed because $1300 was the estimate bcbsil and scripps gave me for the procedure. This $1300 payment was never submitted to bcbsil and did not get used toward my deductible. The hospital did however submit a separate claim for $1300 (in addition to a $8000 claim) which my insurance paid $900 of and now I’m responsible for another $400 since I still have not met my deductible. I feel like I’ve been robbed, and something criminal is happening. I talked to a billing representative at the hospital and she rudely told me she doesn’t see any issue with the way I was charged. A representative from BCBS told me that it is very sketchy for the hospital to bill me like this, but there’s nothing they can do because there is no proof of my payment in their end. Help please! I didn’t realize my employer’s health insurance was so terrible and it barely covers anything at all…i don’t know what to do because I can’t afford these expenses Age-36, location-San Diego, income-70k