Weighty issue?!?

Dear kulula.com we booked a flight to Capetown on the 27th of February 3 passangers thus allowed 60kg( we packed one bag due to convince) of luggage no issues with it whatsoever now when we go back we are charged R 360 extra because our one bag is 5kg over the allowed 20kg per bag? Please would you contact me so that I can teach your company basic math apparently this is too much for you guys to fathom