I must admit, I rarely give in to this kind of thing.

I would far rather force a return at my own cost, than let them keep an item.
It's amazing the number of people that just won't return, as all they were really after was an additional discount.

But it is one of those that you need to take on an individual level.  Some are very obvious that they are trying it on, but others, I will give the benefit of the doubt.