Good luck finding the # 9 envelopes too. Due to it's perfect size for easily dealing with multiple pages of folded 8 1/2 by 11 sheets of paper the #10 envelope is dominant as the main conveyer of documents via mail.
So, we basically have the two main Birthday card sizes, the #10 and a few full document size envelopes to choose from. Of those choices, the editors prefer the #10.
Not living in the US, I have never actually had the slightest idea what the &*@# a #10 envelope was.
Now, should I need to, I can try and supply some sort of appropriate equivalent (I did bring back a wodge of 8.5 x 11 paper from my most recent visit, with the intention of being able to submit to magazines that don't accept electronic submissions, but I didn't get any envelopes).
If you're sending to a P O Box, though, the #10 envelope has a better chance of fitting into the box than a big manila envelope (which may be one reason guidelines might specify the #10 envelope).
Anything more than 5 pages should not be sent in a #10 envelope. A single stamp won't always cover the postage on more than 5 pages either.
On the other hand, a #10 envelope fits perfectly inside a #11, but #11s ARE hard to find. You won't generally find them in every office supply store. Normally you would want to call a print shop and have the envelope special ordered. It IS a standard size for most paper wholesalers, so the special order isn't a big deal... it's just that most print shops don't stock them in inventory.
as always, there ARE exceptions. When I was the Purchaser at a local office supply store, I always kept a box of #11s on the shelf so that people who wanted to include a #10 with a letter, could do so without folding it in half.
I have an envelope size chart on my website:
http://a2zgorge.info/marketing/printing/envelope.htm
I ask because I have a HUGE box of #9 envelopes, and they're pretty. Nice, heavy stock and bright white. They fit perfectly, as previously noted, into the #10 envelopes. I've been using them for inquiries. (As in, "Dear Editor, I'm writing to inquire about the status of my submission...I've included a new SASE for your convenience...") It only just occured to me that there might be some glaring reason editors hate the #9s.
pantros's point about fitting the paper into the envelopes is well stated. I always make sure I fold my letter so it will fit in the #9, in case the editor just wants to scribble "we have no record of this submission" on the letter and return it.
Keep in mind that all editors are failed writers and therefore severely bitter and looking for any chance to dash the dreams of aspiring writers.
That last paragraph was a joke. That describes very few editors and none of the ones that ever read this board. But, if you do not adhere to requested standards, you risk quite a bit if you happen accross one of these type of editors.
I'd stick to using the #9s for other business. Start a pyramid scheme chain letter. Send one out to every state's tourism agency and see how much junk gets into your mailbox.
Use the #9s to remit your payment to the utility companies who are mailing you bills on postcards to save on their own expenses.
On fitting in a P. O. Box...depends on the size of the box. There are various box sizes available...but these sizes vary from office to office.