Two comments;
If identification was not required to pick up an item at the counter of the Postoffice, as long as the person collecting the item was in possession of the notice, anybody could have picked up the package since a standard PO box notification card is easily obtainable.
If the remaining balance of $19.95 was paid when the revolver was collected from the postoffice, Railway Express and/or Seaport Traders should have a record for that payment. Either for the money (and the added service charge) being received at the Postoffice or at least for the transfer of the due amount to Seaport Traders. The evidentiary value of this payment would be massive as it would not only prove that the entire transaction was authentic but also that the revolver was in fact collected at the Dallas Postoffice. Yet, no documents exist that confirms the payment of $19.95 to the Postoffice and/or the receipt of that money by Seaport Traders.