It's more likely that the tips were stolen by postal machines not postal workers. Most tips in an envelope will make the envelope "non-machinable", which means two things. First, you are supposed to pay slightly more postage than normal envelope postage (instead use "package" rates). Second, if the envelope does get into a machine and it is a normal weight envelope, it's likely that the machine will rip the envelope apart and the tip will fall out.

I'm about to send a tip to another FTer and I put it inside two envelopes, including one that is padded with bubble wrap. Hopefully that will take care of the problem.