I just sent out a paperback young adult book that was TOO thin. I expected the book would be cheaper 1st class instead of medial mail because it was thin. WRONG. Unfortunately I needed DC this time. It was a regular mass market size paperback wrapped in brown paper with just around 100 pages.

The post office made me pay $4.95 and it had to be sent Parcel Post. Then they added the DC to equal $5.60 for the 1 thin paperback. WOW!

Never Again! Lesson learned - Add a piece of cardboard inside your package to make your package BIGGER and pay less when you need DC!!!!

Yup, stiff or at least 3/4 inch thick for DC. Either cardboard to stiffen if up or stick something light in it, like a couple packing peanuts,  to fatten it up.

FYI   $4.95 postage is not Parcel Post. It is Priority Mail. Your book should arrive in about 2 days. :).
 If it's any comfort you probably only spent about 40 cents more for Priority than you would have for P.P. and it got there a LOT faster.