Hi all
Just to expand on this a bit...
I've been using Parcel2go for quite a while now and they'd be my first choice when sending things I'd sold to the USA. As competetion has increased in this sector, other players have come to the market - I also now use P4D.co.uk who are also very competitive as well.
Now depending on what you send and it's weight, you need to check between various couriers and compare to each other and check and compare all those with Royal Mail. If it's something quite 'light' then I tend to find the Post office/Royal mail provide better pricing than the couriers. For anything bulkier or heavier it's usually the couriers that are better. Fedex and DHL usually work out the most expensive - I haven't used those guys to send anything yet. However, the other way around, I've had shipments in from US/Far East and TNT have proved cost effective and in one case Fedex were just about the cheapest from Hong Kong. So, I always spend some time comparing to get the right/best price.
Hope this helps - Cheers, DtM! West London & Slough UK!