Do exactly what you did and then if anyone grumbled anymore I would shame them into silence by asking if they would all be as honest as me. I did something like that about a year ago I went in for €15.00 worth of petrol and wound up getting €20.00 partly my fault becuase I was chatting went in and paid the extra €5.00 the look on the cashiers face was priceless he asked was I sure becuase it was his fault too and I said I was just shocked to the core that someone was as honest as me!!!!!