Post Office special delivery maths

Monday 9am and I am already wound up: a Post Office cashier spent 5 minutes arguing with me about how to give me change to £10.07 for a £5.67 transaction. She kept trying to give me 3p back as she "already had 4p"!!! I cannot even imagine what she wanted to do with the 4p. After a some time of shuffling 3p back and forth under the protective glass, I had enough.  I really do try really hard to be nice but i just HAD to tell her that, as "a cashier", if she can't work out how to give change, she should use a calculator, and that it should be £4.40. She gave me a blank stare; her manager, who had apparently witnessed the exchange, found it very amusing. Well, I just had to roll my eyes to him! Good luck, mate! Me, on the other hand, can only despair...