Whoever told you to use a plastic bag to drop envelopes was insane. First of all, that is a major heat-score and with all the crinkling is about as noticeable as if you'd hopped over there on one foot to drop it in. Secondly, you really dont' have to worry about prints on the outside of the envelope. By the time it gets to security bypass chances are it has a bunch of fingerprints on it from postal employees.Do you already have a record? Prints in they system? If not then there is no need to worry at all, and even if you do I still wouldn't be doing anything crazy like that. If you are that paranoid go see if you can find skin-tone latex gloves and wear them for the drop. Plastic bag is craziness!