I have done the same mistake twice, when renting a car using Booking.com they offer a full insurance, I paid it.

When pick up the car, the staff in from the car rental push me to buy another insurance or to block €1500 from my credit card, as my credit is way lower than that, the only option left, was to pay €300 for a proper insurance from the car rental company.

So, the insurance offered on booking is worthless.