I'm going to guess 500g is a typo and you meant 50g.

Try the postal calculator here:
https://ircalc.usps.com/Default.aspx

Notes: A board backed envelope counts as a package which will be much more expensive.

You need to calculate "based on shape"

You will be looking for "First Class International" in the results which is standard airmail. As the cheapest option USPS tends to make it less visible.

Sorry, I forgot to say! The letters will be coming to Japan, from America. So I need the cost of the postage in US$. As for the weight I am not expecting a lot, hopefully just a signed head shot, so less than 500 grams.

What country is the envelope coming back from?

You'll also need to either know the weight or have a rough idea.

Hello,

I am an ESL teacher living abroad. I am doing a project with my English club where we are writing fan letters. I am working on the S.A.S.E. but I am having some trouble figuring out the postage.

If I were to send a return envelope that is a standard A4/printer sheet/8x11 size envelope, how much would the return postage be for Japan? The club budget is limited (not to mention I do not wish to put my friend out purchasing them for me) so I don't want to end up with loads of unusable postage.

Thanks so much in advance!