Miss Manners: Seems like sanctioned line cutting at the post office

DEAR MISS MANNERS: Please help me with line etiquette when faced with the following scenario.
Person A enters the post office and gets behind five people in line. There is a group at the table, fussing with packages (addressing, taping, stuffing boxes, etc.). Person A finally makes it to the head of the line, but when “Next!” is called, Person B, who was packing, cuts in front. Then, one by one, Persons C, D, etc., also cut, stating they’ve “already been through the line.”
I can see both positions. Person A would say, “I’m 100% ready, and you weren’t. You already received service, and now it’s my turn.” Persons B, C, etc. would say, “I’ve already put in my time and waited in line just like you. I’m ready now, and it’s not reasonable to make me wait again.”
Which is correct?
GENTLE READER: The choice of vendor in your example suggests that your experience matches Miss Manners’ own — namely, that the post office is the last remaining business not to have noticed that by ambiguously placing their tables inside the line stanchions, they are causing fist fights in the lobby.
The irrational, but accepted, convention of lines is that it may advance at any pace — and that everyone in it is therefore prepared. This puts Persons B, C and D in the wrong for shoving their way to the front.
It does not, however, deputize Person A to be rude. An alert Person A may therefore wish to stand in such a way as to make advancement from the table difficult. Otherwise, the best they will be able to do is to say, “Excuse me, I believe I was ahead of you.”
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