Hi. in answer to your query, if your return envelope is about 5x7 inches or smaller, it'll totally be more than enough to cover postage.

I've friends in Japan, & in their letters to me, the printed stamps on the envelopes primarily read 110 Yen. (yes their envelopes are usually 5x7 inches or smaller)

Hope this helps.

I'm writing to an author in Japan, but I can't buy Japanese stamps for the return postage.

I was going to send a SAE and 300 Japanese Yen (approx. £2.50, $3.70), but will this be enough to cover postage?

Thanks!