Late delivery with a bad attitude

I'm a company sending 2-3 parcels a day with fastway couriers, my parcels get delivered 2 days later when I'm told it's next day delivery. When calling the manager, her exact words \I have ten thousand parcels to send and you not my only customer\" bad attitude and an attitude that's not willing to help nor rectify. If you cannot hold a management position... Go find a job as a parcel packer. Terrible service! No leaving me in a position with several unhappy customers! "