Do these add a lot to the amount of postage?  I have an odd shaped audiobook and 2 small paperbacks to mail to one person. I'm trying to decided how to mail them. I know packing them all in a box will be expensive. 

No, they are generally under an ounce (there are also "padded" ones filled with chaff that are much heavier - and I hate them, very messy!)

A box for odd shapes, or odd combinations of shapes doesn't have to be heavy or expensive; it goes at the same media-mail rate.    I have re-used cereal boxes and frozen-dinner boxes.  Any small box might work, with padding from plastic grocery bags.  You could also just take a strip of cardboard, stack them, and wrap it around them.  Seal the ends with paper or more cardboard and lots of tape.  A lot of people use layers of brown-grocery-bag paper.

I use the padded bubble wrap envelopes for many of my 1 and 2 book mailings and the ones I use weigh less than 1 ounce so they do not add signficantly to the weight of the book I am mailing.