In reply to your questions, davidc4430, the cost for each pack of books was $5.00 to post - each pack weighed over 250gm, less than 500gm.   Buyers assume that combining books will reduce the postage, but it doesn't always work that way.  I combine my books in packs to reduce postage on individual items.    I could re-arrange the books in any way to reduce the postage, and putting them all together made them weigh in at about 1.2kg.  Even if she'd be in the same state as me, I would still have had to pay more postage by sending them in 2 separate packs over 500gm.  It was a no-win situation, but she assured me she has bought ten books in one hit and paid only $5.00 postage - must be small books, coming from her own state.   😕   

Which is why I asked the question - is there something I'm missing about postage rates over 500gm interstate?  

As for everyone else, thanks, I needed the laugh today - said with all honesty - you all made me laugh out loud. 🙂   And restored some perspective.