What it costs and what it does
Sunbelt and Herc both publish a rental protection plan at 15% of gross rental charges, which makes 15% the industry figure rather than one company's policy.
It is not insurance. It is a contractual limit on what you owe if the machine is damaged or stolen, typically capping your exposure at a percentage of the repair or replacement cost, with a dollar ceiling. Exclusions are real and worth reading: misuse, overloading, unauthorised operators and unattended theft commonly are not covered.
The certificate of insurance route
If you carry commercial general liability plus inland marine or rented equipment coverage, a certificate naming the supplier as additional insured and loss payee will usually let you decline the plan. That is how most established contractors avoid the 15%.
The certificate has to actually meet their requirements: the right coverage types, the right limits and the right wording. A certificate that names the wrong entity gets rejected on the day of delivery, which is the wrong moment to find out.
When the plan is the better deal
If you do not carry rented equipment coverage, if the deductible on your policy is higher than the plan's exposure cap, or if a claim would move your premium, the plan is often the cheaper answer on a short hire. For a one-week rental of a mid-size machine it is a modest amount of money for a defined ceiling.