https://www.facebook.com/1375705053/posts/10217942304293976/
The website above includes the names, addresses, telephone numbers, and email addresses for the board of governors of the US Postal Service.
You have a mission. Write to them. Call them. Email them. Do all three. Let them know that this is our postal service and that we are embarrassed, ashamed, and mad as hell that Trump is using our USPS for his election purposes. 
This is a war for the future of our Democracy. Pull out all the stops. Gird your loins. Sharpen your tongues. This is not going to end like this. So go. Go now. Do everything you can. Your kids and their kids need your help.