Hi,

a) make sure your credit card covers the rental (primary)
b) can you drive stick (as the M in CPMR is Manual)? Otherwise add it to your remarks
c) as Sixt matches status from airlines/hotels/rentals ....you might be able to get a status match and therefore increase your chance of an upgrade
d) only tip > german rental companies are more detail oriented (to phrase it positively) when it comes to damages > make sure all are captured
e) too late for you but you should have checked if six.co.uk has better rates (sometimes they have / or one class is cheaper than .com while another is more expensive)
f) if you would have rented in a non Airport/Train Station location you would have avoided the Premium Station Surcharge (20% ?)