Here's the results of my research:
Now it may be that there is nothing listed because they charge no cross border fees...but it is always best to check directly with the company to verify policies.
With regards to the toll stickers, I did some research on AutoEurope.com's driving guides: Here's all the info I could find for the countries that you stated you will be in. (Note: re: your question of ..."what exactly they are for"... my assumption is that toll stickers allow passage on toll highways/roadways that don't have the ability for one to stop and pay at a toll booth... but honestly, I don't know and could find no explanation of them on AutoEurope's site. My apologies! :)
Netherlands: There are no toll highways in the Netherlands.
Italy: The 'autostrade' is Italy's toll super highway and toll cards can be purchased at banks or at Automoblie Club d'Italia (ACI) offices.
Switzerland: Highways in Switzerland require a toll sticker that costs approximately US$28. Cars rented in Switzerland have the sticker attached; otherwise you must purchase the sticker separately. The fine is approx US$100 for driving with out a sticker.
Spain: No information about toll stickers, but the site states that "Spain's expressways, the 'autopistas,' are toll roads and are identified by the letter A."
Germany: There are no toll highways in Germany.
Belgium: There are no toll highways in Belgium. The Liefenhoeks Tunnel, located in Antwerp, does require a toll.
France: There are toll highways in France that are labled as 'Autoroute peage.' These highways are marked by blue signs with the letter 'A.'
Austria: Austria requires all vehicles using the autobahn to display a highway toll sticker. Rental companies in Austria charge approx US$0.65 per day for a toll sticker. Stickers are available at the border or gas stations when entering from another country
I hope this helps a bit.