For 2 children in middle school.

And since I am divorced we buy 2 copies of every book so the kids have a copy at both houses.
Silly you say??
The load of books weighs about 15 kg - there are no lockers for the kids books at school so
it's either buy 2 sets of books or once a week drive over 30 km ( 30 for me and 20 for my ex) to get all the books.

I still have to spend another 50 euros or so on art supplies that the kids have to have (why do they need to draw exclusively on special Fabriano paper??)

... and let's not forget the bus --- 150 euros every 3 months.

My youngest daughter , who is in elementary school, should have all books free BUT she doesn't because the teacher decided she wants to use another book so we got sent home a notice that the teacher wants to use another book which costs 7 euros or something - do we agree or not.

If you agree you pay and if you don't agree your child has to copy out of someone else's book.

It all seems crazy to me - There are people making 800 euros a month with a couple kids - how the hell are they suppose to pay for the books and the bus and eat and pay rent?

In high school costs are even highier. When I was at that age, I mostly used second hand books, which I bought from either the "mercatino" at Piazza Vetra, here in Milano, or at the Libraccio. But publishing houses, not to have their earnings reduced by second hand books commerce, long since started putting out a brand new edition of every book every other year. Essentially, the text is the same, but a few pages are rearranged and occasionally a few extras are added. Our teachers always prompted us to use second hands books, they were willing to make copies of the added parts (say a dozen pages out of a whole book) and trusted us to be able to follow, by checking the index, eve if the page numbers werent' the same for everyone. Yet, most teachers are lazy and require children to have brand-new books. You as a mother, and your children as students, will be able to confront the teachers and tell them that you don't care, you (your children) are smart enough to use second hand books because you know how to use an index Obviously, this may cause som fights with any particularly not-smart teacher.
Regarding the 7 euro book of your youngest daughter, I would check with the school respinsibles, and if unresponsive with the provincial Provveditorato agli Studi, because I think that this is not actually legal.

There is a government-set limit on how much parents are supposed to have to spend, but it's been widely reported as not adhered to every single year that I can recall. It would require the teachers to actually coordinate on who's ordering what, and to be more disciplined about the books they required. Ross keeps having to buy books for informatica class that they barely even touch, without getting into discussion about how useful they are!