When you travel, ALWAYS carry a pre-stamped Priority Mail Tyvek envelope. If you get hassled, be nice, and tell them you want to file a complaint, and ask to see the person in charge of the shift.
"I know you're doing your job, I disagree with your position, but I will respect it - this time. After I mail this item to myself, I am coming back and I'll want the names of the supervisor on this shift and hopefully the next time I travel I'll have a firm statement from TSA"