It depends on the weight of the book.  Sometimes it is cheaper to send the book first class and that is when the wrapper prints that way.   I am not sure if there is any way to make it always do that.

If you are not using printable postage from this site, then just put a weight of less than 7 oz into the box on the wrapper setup page. It's cheaper to send a small book First Class than Media Mail, so if PBS thinks the book is small it prints the First Class wrapper.

If you are using printable postage, then there is no way to send all your books First Class.

If you weigh the book and find out First Class is cheaper than Media Mail, you can always black out the Media Mail on the label.  That's what I do or I put a white sticker over it and write First Class.

If you take the book to the PO to mail it, you can request that they check to see which way is the cheapest to send.  Sometimes PBS marks it MEDIA when it's actually cheaper to send FIRST CLASS. My PO is always happy to do this for me.  They will even then either mark through or put a label over what is printed on the PBS label, and remark it for you.  Pat

You can also weigh the book ( I don't trust PBS weight) and look up the cost for 1st class postage on the post office website.  I do that sometimes when I'm just sticking stamps on a smaller book and putting it into my mail box.  If 1st class is just a bit more then I cover the media mail markign with a label and write on 1st class.