I find that I have a TON of really oversized mailers to use for really big books, but I don't have any for smaller books. I've been bending them over and taping them in place to make them work. I've been thinking of buying some of the right size on eBay or something, but I know they're sized by number. My question is, what size "number" works best for what book sizes? I especially need mailers for mass market paperbacks and standard (not overly-thick) trade sized books. The bigger and/or thicker books work well in the bigger mailers, but those smaller books are way too small for them. The ones I have are mostly size #2, so not huge but still too big for a lot of books.

Any input?

The 6 by 9 inch size might work for the books you described. Maybe you could go to Walmart and buy a few of the different sizes and see what works the best.

If you have a Sam's Club nearby, you can get the 6x9 (Size O) for about $5.00-6.00 for a box of 25 bubble envelopes.  These hold a MMPB very nicely.  The Size 2 (8 1/2 x 11) are about a dollar more per box.  They will hold a hardback and a paperback nicely, or 2-3 paperbacks.  The plain envelopes are cheaper, but I've neve bought any so don't know a price on them.  Good luck. Pat

I just get a box of 50 Large Card Envelopes for $5, or 100 for $10.

They're thicker paper than regular envelopes. Some even have self-stick flaps. Just trim down either side and open it up. They're usually the perfect size for MMPB, even the fairly thick ones. (Very thick and I use two, end to end.)

I wrap the book in saran wrap, then just center it on the open envelope, fold over snugly, seal the flap, then fold up the ends. I print out the site label without postage, and trim it down to the return addres and the sending address. One piece of clear tape the long way, covering the address' and folded ends, and you're pretty much done. Sometimes on the thicker books I'll cut a piece of clear tape in half lengthwise and make one round around the top, sides and bottom.

It's 10cents for the envelope, not counting the tape costs. I've sent out hundreds of books that way and never had a problem.

I personally like bubble mailers and I order mine from amazon.

I usually get these for hardback / trade paperback

#2 8.5x12" POLY BUBBLE MAILERS PADDED ENVELOPES, 100 ct

You might want this size??  It works out quite cheap at 100 enevelopes for about $12.00

6x10 POLY BUBBLE MAILERS PADDED ENVELOPES

I've used poly mailer bags (just plastic, no bubbles) for nearly 5 years and I find the 6x9 size works very well for mass markets and some trade paperbacks.  I keep 7.5x10 bags on hand for larger trade paperbacks.  I don't swap many hardcovers or audio, but when I do, I try to reuse bubble envelopes, bubble wrap or a box, to protect the hard corners from poking through or getting bent.  

The key with the thin poly is to mold the envelope around the book with plenty of shipping tape -- no flaps, nothing sticking up for the postal machinery to get their teeth into -- like mailing a brick.

If it helps, I use this vendor, who was recommended to me by another PBS member.  Even if you don't buy from them, they sell only mailing stuff, and lots of it, so you get an idea of the variety available.  They also offer poly mailer combos, where you can get a pack with 25 of this size and 25 of that size, which is convenient.

Don't bend them over, cut them down to size. Even an ounce or two adds significently to the postage cost.

IIRC, size 0 is too small for anything but MMPBs, which I package with the PBS printout mailer anyway so they're of no use to me. If you're finding the size 2 too big, try the 1s - they're in the middle of the trade paperback and some hardback range.

I try to reuse as much as possible, but I ask for mailers for Christmas and my birthday to help supplement my stock.  I also find that Walgreens has decent sales on them from time to time if you just want a few.