1999 Pontiac Montana

1999 Pontiac Montana

I had a rebuilt transmission put in my van. A few days after I got it back, the bottom of my van collapsed while I was driving. The mechanic told me the bolts that held up the carriage were rusted and broken. However, I picked up the bolts off the road and they were huge black bolts, perfectly intact, oily and no rust on them at all. Is it possible that the mechanic took the carriage off to fix the transmission and forgot to tighten the bolts? I know nothing about cars and my mechanic knows that. The receipt says coil kit $322.80. Does this sound right?