Printed postage is not good for foreign customers, because it only allows you a few days to post the item, after which it becomes invalid.
You should be able to purchase stamps online through the Royal Mail shop but lots of US members have had problems with this.
International mail is done by weight only and the world is split into two zones, the whole of the US being one. So the prices I already posted in the other thread are the ones you need. It also depends on whether you will be using First Class or Second Class stamps because one is worth £0.60 and the latter is worth £0.50.
I ship to family in NY & NC as well as fan mail all over the US, is always the same priced by weight only.
I see from your OP you were also asking about sending the whole think US to UK,
http://www.usps.com price calculator will help you with that or you can take your item to a local post office for assistance.