buying stamps to return from the US?

help!!

How do i buy stamps to get an acknowledgement from a editor?

"Authors outside the United States should send disposable manuscripts with a
business sized SAE and proper US postage. US postage can be purchased online
at usps.com from anywhere in the world. No IRCs, please"

How do I know the correct postage? *confused* I'm unlikely to send an IRC
as I wouldn't know an IRC if it bit me.