Charged double to what was advertised

I sent a box from EL to CT Postnet to Postnet. The box weighed 4.8 kg. The advert states that a parcel under 5 kg is R99 Postnet to Postnet. I was charged R199. On querying this I was told the box is big. The box was a medium size, easier carried by me. I now have 2 more boxes to send of simi**** size and weight, but don't want to be overcharged again!