Buying a stamp in the home country is doable for most people. But even then, imagine you are 20, how many mails do you think will you have sent?

For a different country, I'd have no idea. Especially if it's so far away like the USA and I can't locally get a special reply post stamp. What I would have done is to put in 5€ in the envelope and call it a day. The person would probably be happy seeing other money.