You can only follow the proper procedures to complete your side of the transaction.
If you have a tracking receipt that your shipment was delivered to the address provided by the buyer which you must have to legally sell ammo by mail,you have completed the transaction to the best of your ability.
If the buyer cannot get someone he trusts to sign for it,then that's his problem.
Tell him,you have the signed receipt and for him to check with whoever he had sign for it.
This transaction is delivered, signed for legally and over.