“A good many young writers make the mistake of enclosing a stamped, self-addressed envelope, big enough for the manuscript to come back in. This is too much of a temptation for the editor.” Ring Lardner
The agents who still insist on snail mail queries/submissions (and there are a great many of them) will often ask for a SASE – a self-addressed stamped envelope.
Complying with this request is not a problem if you happen to be querying an agent located in the same country, but if you’re in Canada, and querying in the U.S., or overseas, for eg., it can be a real pain to try to find foreign postage.
Remember, the receiving country won’t use your country’s stamps; it requires its own. Although I did get one of my SASE’s from the US mailed back to me (rejection letter inside) using the Canadian postage I’d wrongly stuck on it. I guess the sorting machine didn’t notice my mistake
Should you include a SASE at all?
In my view, having done so several times, it’s a complete waste of money. If an agent wants your manuscript, they’ll call you or email. No-one sends a request for a full or partial by mail these days: it’s simply too slow. If you don’t hear from them within a reasonable time, they weren’t interested.
And so what you are doing by enclosing a SASE, is giving an agent the means, at your expense, to send you a rejection letter. You will never get a SASE back that has good news in it, so unless you’re looking for expensive wallpaper, why pay good money to be let down?