£35 is quite cheap for airmail delivery. DHL charges around £60:
http://apps.dhl.co.uk/cgi-bin/calc.cgi
(Parcelforce Worldwide has comparable prices for their 'Express' and 'Priority' services; I assume that you're quoting from their 'Value' or 'Economy' rates).
Air fuel is very expensive. A 747 needs nearly 100 tonnes of the stuff to get across the Atlantic, with every gram of the load adding to the quantity of the fuel required. (That's why airlines charge so much for baggage). In an age when it can easily cost you 50 quid to watch a football match or to buy a West End theatre ticket, £35 to send something across 4000 miles should be regarded as a bargain!
Unless you know someone who is travelling to the USA (who could then post your parcel upon arrival) I very much doubt that you'll find any cheaper option.
Sorry :(
Chris