Unexplained charges

We received one consignment from Courier Guy, but we are charged twice. When I challenged them, they answered that we are billed for both, as if we did not know that already. I gave the numbers of both consignment notes, one for economy one for overnight, but they are not interested in the explanation. It also does not help to ask for proof of delivery, because they scan old signatures onto their system. without a physical signature. The irony is that the document that accompanied the parcel clearly states it is economy delivery, but the second document for the same delivery is overnight charges.