Hi, I am a new member to PBS and yesterday mailed my first book.  At the post office counter, where it was my intention to purchase the $2.13 postage, the clerk informed me that "did you know that for $.02 more you can send this first class"?  Obviously I did not know this and just added the $.02 and sent the book first class.  Can I expect any problems or issues with having chosen this over the media rate method?

Far from it, for 2 cents extra, your book is likely to arrive several days earlier!

it depends on the weight. Up to 7oz, it's cheaper (or $.02 more) to send First Class parcel rate. After 7oz, media mail is cheaper.

First class is generally faster. :->

Here's the link to the USPS postage rate calculator:

http://postcalc.usps.gov/

I'm lucky enough to have a small postage scale, and it's very enlightening to see the differences that an ounce or two will make.

You are free to send books in any manner you want.   Media mail is just one of the ways.