Apparently, I was 87 minutes late in picking up my car and Sixt only guarantees a rate for 60 minutes. As such, I was switched to a 2,100km limit from unlimited and went about 600km over as a result. They seemed to have decided to charge me about half the regular overage mileage rate for a total of about 72 Euros extra. The 50 Euros for the AdBlue was credited without incident. Paying 72 Euros extra for an originally unlimited rate plan is ridiculous as it'll just encourage people to use another rental agency if they're over 60 minutes late.