[thebmwjunkiee]

If you're familiar with paint correction, I'd try that first. If not, maybe have a reputable detailer give it a shot? Worst case you spend 100-200 and still have plenty left over to replace the parts. Make a "wanted" post in the classifieds for a rear bumper and diffuser, you'll get them much cheaper than from the dealer and can just get them painted/installed. You could probably even upgrade diffusers rather than replace the stock if you prefer. I'd much rather try to buff it out or buy used / upgraded parts rather than just hand the 1800 over to get OEM replacements

I'd be very surprised if it did any damage under there though. Do you have comfort access for the trunk, and if so is it working fine?