There are no outside companies that offer insurance of this type. Penske will tell you up front what the cost will be; it's generally a dollar amount per day and usually not outrageous. It's well worth the investment; should anything happen to the truck, they deal with it themselves! You might be able to get a quote on the rental as well as any insurance cost on their web site:
http://www.pensketruckrental.com/persona...
good luck!
Since neither your own insurance or your credit card company will cover, and your only option is Penske, you will have to call Penske and ask, then you will know and can sign up for it.
Good luck on the move.
You are correct that your auto insurance, and credit card will not cover commercial trucks.
Your rental contract should include a minimum amount of insurance coverage, but I would suggest that you price Penske's Supplemental Liability Insurance (SLI). It offers you up to one million dollars of coverage if you are involved in an accident while driving a rental truck. To inquire either ask your local Penske agency, or call their corporate office at 1-888-990-5415
PS: This nis a moving tip from a former professional mover. Load your refrigerator FIRST so it is in the front of the truck.