Is there a way to refuse a USPS shipment?

[rgG]

I ordered a small item from Buy.com and then decided to get it locally. I tried to cancel the order, but it was too late. They say I have the option to refuse the shipment, but It looks like it has shipped USPS. It is small enough to fit in the mail box, so I don't see how I can refuse the shipment if I don't have to sign for it. Can I take it to the P.O. and "refuse" it there or is there another way?

If I can't refuse the shipment and need to send it back, what is the cheapest way to do it and still have some sort of tracking to prove that I indeed shipped it back. I order quite a bit of stuff online, but I almost never return things, so I am not sure what would be the best way.