The chain of custody should cover the printing of the ballots at the printing facility, during transportation, in the office before mailing, and when received. When developing your chain of custody assume that you are going to have a court challenge and that you will have to prove that your ballots were constantly in the possession of an election official or a sworn officer and that there was absolutely no time when anyone could alter any ballot, either before it was cast or afterwards.

https://www.eac.gov/sites/default/files/eac_assets/1/6/Chapter_5_Ballot_Building.pdf