Theoretically, there would be a professionalism in the ranks of the USPS that would call for them to transmit all correspondence duly paid for. However, in reality, those union thugs will look at the poster's of such correspondence and if they disagree, they will ditch it.

I am a rather prolific poster of letters to the editor in our local paper. I have no doubt that the postal carriers and postal workers in the local office know who I am, and my political leanings. It would not be difficult for them to take my ballot response (since I'm usually out of town on election day, I nearly always vote some form of absentee) and dispose of it. I nearly always bring the ballot in personally, so that it is delivered directly to the clerk of the town. Does this ensure that it isn't then disposed of? No. But I can at least after the fact check to see if I had voted, and if not, challenge where my ballot ended up. Not foolproof, but better than nothing.