Sixt bait and switch tactics are amazing. I'm currently in a rental from them and the rental agreement I have does not include the prepaid fuel, but if I go into my booking it does. I'm not upset, because hilariously the price they are charging me for it is how much it would cost to fill this car up right now (maybe a few bucks more), and they did upgrade me from an XCAR to a BMW 840 grand coupe and on top of that let me pull a brand new car seat out of the display case because I didn't like that the used ones weren't bagged despite being allegedly cleaned, but still crazy that two different rental agreements exist.
Oh, and FWIW I got my rental agreement electronically (but this time an actual rental agreement), so I think Sixt is in fact moving away from handing you the fistful of paper.