How would the devious Biden supporters in the Postal Service know that the vote was for Trump?
Well they would know it was a vote , they would know that the sender would not know whether it was delivered or if a few thousand votes from certain locations were not delivered , (unlike normal mail) if you deliver all the postal votes from one location but not from another it is possible to skew the vote.
They should never have allowed postal votes to continue to roll in after voting day and had a cut off date before polling day to allow all postal votes to have arrived.
If anything though why did we make such a big deal about the integrity of the e Voting machines when you see the shambles of mail in voting in the US almost 2 decades later.