Hello - would like your opinion on listing items which include postage (ie FREE POSTAGE)

Is FREE Postage worth it?  I think as a seller we have to pay eBay some 10% on the entire amount - ie a book @ $10 including free postage is going to cost some $8.65 with an eBay postage label which means you pay eBay about $1.10, postage with eBay tracking $8.65 + $1.00 for the envelope, so you pay at total of about $10.75 selling your book.  Am I correct? Obviously more expensive items are different, but should I stop selling  cheap books and just concentrate on higher priced ones?

Please don't call me an idiot because I work hard for my points, but constructive criticism welcomed 🙂

Jenni