Is it critical that the Witnesses be at the Post Office for the mailing? Yes. See also: What do the Witnesses do at the Post Office? Do I need to use Zip/Postal Codes on the envelopes? Updated on November 4, 2018 Was this article helpful? Yes No Related Articles Why do I use a Certified or Registered Mail Return Receipt card? What Post Office supplies do I need to pick up? What mailing service should I use? What is the recommended ordering of pages/documents in my mailing? How many copies does each Respondent receive? Do I make copies of my documents?