I was putting together queries today - those that requested pages, synopsis, etc., went into a large envelope with an SASE (stamped - yes, I checked) and the other materials. Those who requested *just* a query, I put into a standard #10 envelope, with the SASE folded around the letter. I can completely understand agents not wanting to read many-folded manuscript pages, but do I really need to spring for a catalog envelope plus postage for a single-page query letter?
Catalog envelopes are needed only if you are mailing more than three pages and you want the pages returned (why you would want this is beyond me, but it's your call).
You put a Biz envelope in for a query that is just a page or two. Very very smart and savvy snarklings understand that biz envelopes come in two sizes: #9, and #10. You put a #9 inside the #10 for your SASE. You mail the #10. Don't tell me you can't find #9 envelopes.
For everyone rolling their eyes and shrieking, just use a #10 folded neatly.