This sort of thing is extremely common across the industry and Sxt is far from the only one who engages in it. We always recommend that renters review the costs on the rental contract before signing to ensure there are no surprises.

As for your complaint and credit card dispute, you are welcome to pursue both, but Sixt will likely respond to the dispute that you signed the contract with the added insurance. You may still win the dispute but it's far from assured.

Also keep in mind that disputing charges with rental companies can land you on the Do Not Rent list. If you don't plan on ever renting from Sixt again, that's not a big deal, but just something to keep in mind.