I received a book I ordered today.  The label was handwritten on an envelope and there was no postmark.  I live in Florida, the return address is in Pennsylvania.  The book had stamps on it - four 20 cent stamps and two 2 cent stamps.  There is no way any of the stamps fell off.  That means the sender sent a 7 ounce book with 84 cents of postage and it went first class and I received it in less than a week.  Why am I spending over $2.00 mailing the same size books?  Was this just a fluke?

This was them using incorrect postage and the post office not catching it anywhere enroute. 

Definitely NOT the correct postage. You were lucky the USPS didn't catch it and they arrived postage due.

Why am I spending over $2.00 mailing the same size books?  - Because you are sending them with the correct postage and they are not? It sounds like they are sending books using the flat letter rate which is NOT the correct rate for any books (other than maybe those thin floppy kids books) and could easily arrive postage due should a knowledgeable postal worker look at it closely along the way. If they are sending packages using a letter rate intentially, it is postal fraud.

If you have not filled in the book survey yet, please be sure to note this in the comment section.  That gives PBS a history if this member keeps doing this. 

This person broke all kinds of postal rules. I don't think you are allowed to mail something that weighs 7 ounces using the first class letter rate. Maximum weight is 3.5 ounces. Maximum thickness is 1/4 inch.