2000 GMC Sonoma oil leak

What is leaking oil? Need to figure that out first.
Then I can comment.

The converter shield may have been affected from putting it on the lift, but that might be a stretch. I doubt there is a relationship there. That comes down to public relations. IF it was my shop I'd thrown a $2.00 clamp on it and make my customer happy if we had a good relationship. IF not, I might charge them. But that is my view. I take care of my customers, but weed out the problem child customers. Not to say you are in that category. Just saying. Some places have a policy they follow that tells them what they can charge on not charge for. It may come down to the paycheck. If they are paid on commission. Then you are more apt to get charged I would expect. In any event, this is unrelated to the job you paid to have done.