To prove to have paid this fee, the toll stickers have to be put onto the inside of the front windscreen. Once removed, they no longer are valid, so each car on Austrian Autobahns needs its own toll sticker.
To validate payment of the fee, toll stickers must be placed on the inside of the front windshield of every car using Austrian Autobahns.  Once removed, these stickers are no longer valid and each car needs its own unique sticker.