Am I liable for lost keys in USPS mail?

I recently resigned from a job which I had keys for. So I sent them through USPS in a small envelope (I know, I shouldn't have, but I did it before reading you shouldn't and should rather use a menela padded envelope as these are hand sorted) and my ex boss said he received an empty envelope (did not state how the envelope was) and also said if he has to change the locks he has to change 100 at $8.00 each. So I'm assuming they either got lost from the envelope tearing while going through the sorting machine or someone stole them, which is very slim as only 2 people would have came into contact with them as it is automated, the person picking it up, who didn't because my family knows him and the person delivering it. My question is, am I liable for the loss even though a third party delivered it? Can they send me the bill, and do I have to pay it? Can they bring me to court? Could they some how make a claim with the post office and get compensated?