Made to pay even though I checked in online and arrived 40min before

Today my experience with Flysafair was bad. I arrived 40min before boarding, my bag was already checked in but I was told to pay R300 and something they later changed it to R155. I dont want to mention the ladies names who basically cornered me to pay the amount even though I told them I don't have the means because I checked in online and had just paid an arm and a leg this weekend taking my niece to varsity I even cried and they insisted that I pay atleast R155 or I'd have to leave my bad and later courier it