let's assume I land on the post office leave an assistant and take the stuff. In the next round I just use my two moves to jump 1 tile forth and then back to the post office pick up my assistant and get the next stuff again. Next round I move 1 tile forth and 1 back leave my assistant ... and so on. Is that allowed? And if what's the point of the card to not move your merchant? I couldn't find a statement in the rules that you cannot move back to the tile you are comming from. In general it would mean that you just stay on one tile and pick up and leave your assistant for as many rounds as you want.
No, you can not visit the same tile twice in a row.
You could visit the Post Office, drop an assistant there, and then on your next turn, move one or two spots away, and then on the turn after that go back to the post office to pick up your assistant and perform the action again.
But you can't do it in two turns in a row. As you noted, if you could, why would you need the "move 0" bonus card?