More information is here:
https://www.hertz.com/rentacar/reser...NS&EOAG=FRAT50
It appears the real issue is
1) Taking a very expensive car outside of Germany (there is a list on the page)
2) Going outside of Germany when you don't tell them
Since you are not intending to travel outside of Germany, then you should be fine. I would just mention that you'll be near the border.