I also had a faulty DMD in my MET Pro and Stern sent me a new one free of any charges (including shipping). I've also received several other parts free of charges. Stern's QA isn't great (to be polite, but their product support makes up for it. The distributer shouldn't be charging you anything. You should deny the credit card charge…period. At most, a modest shipping charge might be reasonable.
For good or bad, these machines are now being targeted at pinball fanatics/hobbyists (who don't mind dealing with all the NIB "speed bumps"). When I started in 2004, I didn't even want to remove the glass and lift a PF…now I'm restoring machines and doing all my own maintenance and repairs.