Is it fraud to accept too much change in a shop?

It's not fraud, but it probably is theft. Fraud is "wrongful or criminal deception intended to result in financial or personal gain". Fraud would involve inducing the shopkeeper to part with the money, which is hardly the case with too much change. But knowingly taking too much is theft, defined as the unauthorised taking, keeping or using of another's property. Legally speaking this does not extend to accepting that extra coin accidentally, for the act of theft has to be accompanied by a mens rea – a "guilty mind" – which includes the intent to permanently deprive the owner of the rightful possession of their property or its use. So if you know you have done it, and you know this is wrong (otherwise you would not ask the question) then it is wrong. Irrespective of the law, staff are often forced to make up shortages in the till at the end of the day from their own pockets. Hand it back.