Poor customer service

Poor customer service, flysafair port Elizabeth.I had a small bag weight 6.5kg .I was told that I need to pay for the language because it doesn't comply with a certain dimensions.Funny enough I have traveled several times with the same bag,I never paid anything at Flysafair .even on the payment options that I had to select online there's was nothing written pay for noncompliance with dimensions, however I ended up paying for 20kg bag wherein the bag was only 6.5kg.