Never do this:
I recently mailed you a query letter regarding an MS I had completed _______________, for which I enclosed a SASE. It has just come to my attention that my secretary (me) forgot to put a stamp on the camp. Please find enclosed appropriate postage.
And enclosed is a stamp.
Um. That’s very nice. But I am not going to label this stamp with the MS title and leave it on my desk until the MS shows up. Especially because there is a good chance that I already read the query letter, found the stampless SASE and trashed it.
If you realize you’ve done something like this, don’t send me postage. Just resubmit the MS.
Oh well. Free stamp for me, I guess.