I got my first big order-I've never had anyone ask for more than 2 books at a time before. I've got a request for 6 books, and I was wondering if one of those small flat-rate boxes would be the most cost-efficient way to mail it. Could I get some advice, maybe from some boxers? I've got a couple days before I have to accept the request to figure out how to mail it.

How big are the books? If they're mass market paperbacks that aren't too thick, you can wrap them tightly in plastic so they form a brick and then wrap that in heavy paper (like a cut up shopping bag). If they're not roughly the same size, or they're hardcovers, then you need a box. I save and reuse Amazon boxes for these occasions.

A medium sized Priority Mail box costs $11.35, so it's cheaper to use your own box and send Media Mail.

They're bigger than MMPs (are trade paperbacks the next size up?). They're not terribly thick, but they're also not all the same size, so I do think a box will be necessary. That's good to know about Media Mail being cheaper than flat rate.

Yes, Media Mail is much cheaper than the flat rate boxes.  Half a dozen paperbacks don't weigh much.  Couple days ago I mailed 4 or 5 books in a small box, they weighed 3 lb. 7 oz. and cost $3.73 to ship Media Mail (plus DC b/c I buy counter postage).  So, under $4 when a flat rate box would have cost over $10.

Just keep in mind, do not recycle boxes that originally contained items banned from mailing (such as liquor, lighter fluid, fireworks, etc) and do not attempt to reuse Priority Mail packaging for different mail rates.

I also prefer to re-use the Amazon boxes (when I have some on hand), but have also used small coffee, candy, and snack cartons (rescued from grocery store on restocking day).

I have used a poptart box for... 4 books I think, and a cereal box for 5 or 6 books.  I think I mailed 7 out in a shoe box too! Otherwise, I also save amazon boxes to mail books in. :)

A box is definitely a good idea with books that are different sizes, just not one of the flat-rate ones -- they have more expensive postage charges, as others have mentioned.  Maiing them all in one parcel, using Media Mail, would be the most cost effective.  You might consider springing for PBS Delivery Confirmation for this order too, if you normally don't, since 6 books are at stake, that's inexpensive insurance for your credits.

PLEASE do not use a manilla envelope-- those seem to be what break open most often when multiple books are placed in them.

I just placed an order for 4 books. Was I surprised when one book arrived last Friday by itself though sender had planned to mail them together.  I PM'd her and she said she had mailed that heavy double paperback alone (it was 2 novels printed in one book).  so I respond, Ok, will look for second pkg that should contain 3 books. GRRRRRRR-- come home from long weekend and there is an empty manilla envy in our mail placed into a large clear plastic sleeve by the PO with a sticker that says "received empty".  It appears the envy was cut open (maybe for media mail inspection and NOT resealed). It is definitely not torn open which I've seen happen several times with PBS books as well as other orders I've gotten from online vendors.

Actually media rate is only cheaper once the package is over 6 oz.  You can google or bing the USPS price list and get the rates.  Then print out selected pages.  The last time the rates went up I printed out pages 1, 5 and 59.  That gave me the first class rates, the media rates and the details of what is considered large envelopes and packages, etc.  Remember you cannot send anything over 13 oz. from your home.

Here's a tip for sending in a box.  You can use grocery bags like the ones you get from Walmart or Kroger and wrap the books in those and use them for cushioning too.  You want to make sure that the books are wrapped good just in case the box gets wet and put extra cushioning in so they don't move around and get beat up.  Congrats on you first big order.   It's a great way to save on postage!