Is there an option to mail a book either way or only the cheapest?  Sometimes I have to use an extra stamp or two to mail something and I was always curious whether I was actually paying 1st class but mailing Media....if so, I'd just rather send it first class since I'm paying that amount....

The PBS label will use First Class if it's cheaper or only slightly more then MM.  MM is a flat rate no matter where you ship it but 1st Class ( I think) is based on where it's going as well as weight and size. At least for packages.  So if I have a book that's not going far, I'll look it up on the USPS postal calculator. Sometimes 1st class is only a bit  more and I'll add the extra postage. 

You have to be more careful if you're opting for 1st class, because 1st Class prices are charged per oz & any fraction of an oz is considered a whole oz.  If the book weighs 7.2 ounces, the postage rate is the same as it would be for 8 oz.   They're nitpciky about it too, so you've got to have an accurate weight if you're printing postage at home or using stamps.  If you use insufficient postage, it will either be returned to you or will arrive with postage due - either of which aren't good:P  1st Class also has weight and size restrictions; you can't send a package or large envelope weighing over 13oz 1st class - it has to go priority.  The size limitation is based on package dimensions; a package can't be larger than 108 inches in combined length & girth, and I don't recall the exact dimensions for large envelopes, but I know they can't be more than 3/4 inches thick.  Media mail prices however, are based on 1lb increments, the minimum price being the same from 0.1oz all the way up to 1lb.  Any fraction of a lb after that takes it up to the next lb, and so on as you go up, so aside from being the cheapest rate, it's a lot less hassle than 1st class:P  I don't know anything about a size restriction, but I think the max weight is 70lbs.  You're very unlikely to ever mail anything to a PBS member that weighs that much, but I suppose it's possible with a very large BOB swap:P

hhhhmmmm at my post office they told me to round up or down to the nearest ounce for first class.  So far I have not had any problem (It is very hard to type with my fingers crossed).

Rebecca

It should always be round up for 1st class.  That's not surprising that someone would tell you that though.  I find you get different answers to the same question, depending on what station you ask at:P  I don't have the link handy, but I'm sure this info is on the USPS website somewhere.  I've seen it there before, I just don't recall exactly where.

*ETA - It may just be that they're really nitpicky about it when you use printed postage.  I drop things off at mine all the time, and had one that was off by like .3 oz one time and you'd have thought they caught me robbing Ft. Knox:P

I usually take my books to the post office.  One time I had the clerk compare the meda and 1st class rates for a particular book.  For that book it was only 12 cents more to send 1st class, so we blacked out the "media mail" on the wrapper with a marker, and I sent the book first class.