I researched this a while back - USPS Surface Mail is pretty much the cheapest for large heavy items. Other carriers get pretty expensive for bulky items. One to try is Airborne, the other is
www.eaglegl.com. But BEWARE - anything "airport-airport" carries huge charges at Heathrow - something like £80 on top for several types of handling, customs (several, including non-account charges), and fuel.
I had a 54x6x8" parcel sent to me by Airmail Parcel Post (4-10 days, cost $70) on 25th March and it arrived last week with a lot of stickers indicating it had been sent to Germany by accident! Quite why it took them so long to figure it out I don't know... I was just about to get the sender to file for insurance.
Gareth