Have you tried the price calculator at www.usps.com?

It's always worth considering overpaying slightly, especially if you are using fixed value stamps. That way, if the item takes a long time to come back, there could be price rises in the meantime, and you have a good chance of still being okay with the amount of postage you used.

Can anyone tell me how much in dollars it costs to send a 10x8 hardback envelope from US to UK, under 100g??

I've been putting $4 of stamps on but wondered if I'm using too much??

Thanks