Not really overcharging, but might be related.
My husband went to the Earlsfield PO to take out some money. He took out £100 and it was only when he got back that he realised that he only had 4 X £20 notes instead of five. Because he noticed immediately he went straight back and they had film of the younger man counting out the money and he could prove that they owed him the extra £20. If he'd noticed the next day though that film might not have been available, I don't know how long it's kept for. As it was he was late for work and not very happy!
It's probably just an accident but at best indicates that they're sloppy. Watch carefully!