Hi,
My items are quite flat, so I can get them through the Size Guide as a small parcel - however even my lightest/smallest items cost £3.50 this way through standard Airmail. There is an "aim" of 5 days on this service, but don't take that as guaranteed - it varies from 5 - 21 days (average is 10 days)
I know that if you want it faster you have to pay more, it seems to take 2 - 3 days to get to the US (that's assuming customs don't hold it once it gets to the US) and cost a minimum of £49 no matter how small it is.
If anyone has a miracle, fast , cheap service I'd be really interested to hear too! :-) Good Luck
P.s. Just found
http://www.parcel2go.com/ - not sure if that'll be any use? :-)