Pep Boys, Chino, California Striped the oil drain plug threads and did not tell me Chino, California

On 11/25/12 I took our PT Cruiser to them for an oil change.  They did the work in a reasonable time. The next day I noticed a drip of oil on our driveway about the size of a fifty cent piece.  I guessed that maybe the mechanic just forgot to tighten the drain plug.  I jacked up the car and got under to tighten the oil drain plug.  Guess what... it was completely stripped out.  I went back to Pep Boys.  The service manager on duty had a mechanic put the car on their lift.  He confirmed that the plug was stripped out but tried not to take responsibility.  They put in a bigger plug and told me that the oil pan was going to need to be replaced.  They want to charge me $ 200.00 for a new oil pan.  The mechanic who originally worked in it was not honest enough to tell his boss that he ruined an oil pan by stripping the drain plug.