All overseas mail goes "by air" so any difference in the cost (from domestic mail here in US) would depend on what country it is going to. I think.
$2.10 if you buy a prepaid airmail letter from the postoffice... not sure how much an international stamp is if that's what you're asking. A postcard is $1.80, so i'd imagine $2.50-$3
15 Thai Baht for international airmail postage, which is currently (April 2013) around 50 cents USD.
£1.28
how much is an air mail letter to Sweden
From where?
90 cents
32 cents
3 cents regular 6 cents airmail
24 stamps ridiculous ha i had to do that
Same way other trading countries do it. Much of it goes by ship, trade with neigboring countries may also go by train, smaller items may be transported by plane.
It depends on what kind of goods, and the quantity of goods - and what other countries!
It depends on the currency you're using.