The overpaid line is good. That's happened a couple of times where I've charged 85 and they've given me 90 and told me to keep the 5, which is nice.
I use a similar line (though genuine with my response) with timings - I always check the time before we start and if anyone moans "oh, you're not a clock watcher, are you?" (which I'm not) I simply reply, "No, I just make sure that I never cut a booking short. Always want to make sure you're getting your money's worth!" Which is true, but also reassures them, whilst I have an idea of when the booking should draw to a close.