Cheapest way to ship to the US??

I recently sent a small box (weighed about 4lbs) to the US using Royal Mail...it cost me 18GBP!!! I was appalled. The stuff inside wasn't even worth that much money. It was just some odds and ends and a small b-day gift for my sister. The surface mail was only 2 GBP cheaper so I just went with the airmail.

What is the cheapest way to ship things (small packages) back to the states? Is there a cheaper courier to use than Royal Mail?

Thanks!!