I think at least it is worth a shot at opening it at a post office. Choose one locally that is not all that busy and go in at a quiet time, then they may be more inclined to help you or be a witness.

Then upload your stat dec and comments to the dispute. Do as countess suggested.

It may be that going to VCAT is a wase of time but the fact you covered yourself with the box opening and stat dec plus the threat of VCAT may be enough to make the buyer back down.