Ridiculous Pricing!!

As a small business owner I make often use of Courier Guy to ship parcels to my clients. Many times, I truly feel like they overcharge but non the less, I still keep using them. But today was the last straw, I literally shipped a new born baby grow and a small tumbler, together they don't even weigh 0.25kg, the quoted price was R149 for overnight shipment, which I was fine paying for. Got an email, my account is in arrears because they charged me another R50 because the "Volumetric Weight" is 3kg according to them. It is a NEW BORN BABY GROW AND A SMALL TUMBLER!!! I am completely done with making use of their services.