We're pretty big fans of "killing people with kindness". There was one customer who didn't pay upon checkout, which we request, and then didn't respond to contact. We sent them two notes; the first was "oops, you forgot to pay", and the second was "hey, we can hold your package another week without payment in case you need more time, but then we have to cancel the sale so someone else can have a chance to own this product."
we got a response three or four days after the second convo...the buyer was terribly sorry, paid right away, and was a happy camper. if we had pitched a fit, I'm afraid we could have bad feedback. The best response is maybe be nice, but be firm. What do you think?