Try to get stamps from the destination country and use those.

Some are easier to get than others, Canada for example sell them online through their website and USPS through their eBay store.

viewtopic.php?f=117&t=30702&start=75

As for prices most of not all Postal agencies have price guides and easy to use calculators online

You will need a rough idea of size & weight of envelope coming back to you.

Hey guys, so I live in Brazil and for some reason the brazilian mail post office is not selling IRCs right now, they show up unavaliable on their website and the attendants dont know when they are going to sell it again. I was wondering if this has happened to any of you international collectors, and if yes, any tips?? Any of you know how to buy the IRC online?
i tried searching for the online websites of mail post offices of countries close to Brazil so that the shipping cost wont be too much and also i wont take too long to arrive for me. But sadly none of them sell the IRC online, only in the physical store (i checked uruguay, argentina and colombia).
Personally I would rather like to avoid having to put money inside the letter for the reply, because brazil has a rule that forbids sending money inside letters lol but i think that i could use it as a last resource... If you guys don't mind me asking how much money do you put in your letters? i honestly have no idea about the prices of stamps in the US

Any help will be appreciatted!! Thanks a lot