Does PBS not use media mail anymore?  I had to use first class on my last mailing with PBS postage.

Why?  There is still media mail. 

Somtimes, it is cheaper to use media mail rates.  That is $2.38 for up to 1lb.  First class is cheaper up to 7ounces which is $2.24.  I even use first class for 8 ounces as it is only .03 cents more than media mail and gets there faster.  For 8 ounces, it is $2.41.  Go onto usps.com and look at the rates.   

What Priscilla said. 

If your outgoing book (according to the database) weighs under 7 oz. it's cheaper to mail it First Class, so the PBS label generater chooses the cheaper rate.  Over 7 oz. is cheaper to mail Media Mail, and your mailing label will be printed to reflect this.  Occassionally there are errors in the data base regarding book rates.