You also need to look closely at the wording of their baggage policy. For example Jetstar say that for every kilogram over the limit you pay XX$. So if your bag weighs in at 20.5 kg and the girlie tries to charge you excess, then ask her to read her policy carefully. You aren't 1 kilogram over therefore have no legal obligation to pay any excess. But once you go over 20.99 kg you have to pay.