unless you want to lose money on mailing, (which I often do but I want people to have certain books ):

You really can't cover shipping unless the books are very small if you are doing a 1 credit deal. 

I Guestimated this based on deals I send out--hope this helps somebody 

USPS Media Mail rates*:

Up to 1 pound.......................................$2.41 (1 book or 2 small light books)

but You can probably send out those little cozy or romance books  maybe 3-4 and still make it under $2.41.

More than 1 pound, less than 2 pounds....... 2.82 (1-3 books if they are paperback, more if romance or cozies)
More than 2 pounds, less than 3 pounds..... 3.23 (about 4 books)
More than 3 pounds, less than 4 pounds..... 3.64  (about 5-7 paperback books if light) (probably more like $3.83 not $3.64)

More than 4 pounds, less than 5 pounds.....4.05 (about 5-7 books, may include a hardcover) (I just did this amount for about 3 normal sized paperbacks and 2 hardbacks)

More than 5 pounds, less than 6 pounds.....4.46 (about 8-9 books, all paperbacks, or mix and match as required to meet your postage needs)

and so on...........
More than 6 pounds, less than 7 pounds.....4.87
More than 7 pounds, less than 8 pounds.....5.26
More than 8 pounds, less than 9 pounds..... 5.65
More than 9 pounds, less than 10 pounds....6.04

Warning: these rates are changing in January.  I *think* the new one-pound rate is $2.47, and each additional pound is 22 cents (rather than the current 21), but I have heard this only second-hand.

This is why it is helpful to have a scale.  If you're only a smidge over the borderline, you can also remove a bit of packing paper or something. :-)

I do a lot of deals, although I stay away from one credit deals. I find that a reliable rule of thumb is that 2 "average" size paperbacks (350-400ish pages) can almost always go for less than 1 pound (if you wrap in plastic wrap and paper, like I do).

After the first two, I usually figure that adding 3 more paperbacks will be roughly a pound. So, 5 paperbacks almost always goes for (just) under 2 pounds. 8 will usually go at the under-3 lb. rate.

I do not add a lot of weight with my packaging, though. I mail up to 12 paperbacks without needing a box. (Yup, possible to have a very secure package without a box).

I found this on USPS's website about the January price increases: http://about.usps.com/news/national-releases/2011/pr11_factsht_pricechng_1018.pdf

No mention of Media Mail specifically, just "standard mail" and "package services".  So who knows if media is included in that or not.  Everything else I found didn't mention media mail either, and they were all 3rd party articles (nothing from USPS directly).

The interesting thing is, they are making DC free for some services apparently.  DC for priority mail packages printed online has been free for awhile now, so I'm curious to see if they start making it free for packages mailed from a counter, or if it will apply to other online postage (like first class or media mail maybe?).

Changes take effect Jan 22.

Media Mail is a subclass of Package Services.

I will try to copy the grid showing the proposed Media Mail rates, but the formatting might come out wrong.

I was afraid of that!  The weight is squashed right up against the dollar amount.

Anyway, you can go to Postal Explorer, pe.usps.com, and on the right side, there is a link to *New* Jan. 2012 Price Change Info.