Perhaps you missed the first comment by C210N. It includes this statement:
The mandatory “shall” language in Article I, § 4 with regard to prescribing places for holding elections has no significance if states are instead free to allow voters to vote in federal elections literally everywhere and anywhere. Everywhere is not a prescribed place. It is a total failure to prescribe places for holding elections to prescribe an unlimited number of places for voting.
Has your particular post office box been explicitly prescribed as a voting place by your state legislature?
I think so. I know voting locations are, and there are hundreds and they are interchangeable in AZ, so you can vote at any location.