Cash advance from credit cards is very expensive. The problem is that $9 for $91 during 5 days is even more expensive. (It's more than 60% in a month!) When you are charging more than the credit cards, it's too much.

The problem you have is that you must charge more because you don't have enough information and leverage to ensure the users will not commit fraud. Banks know the history of each user, how much the earn, have a good lawyer team, so they can charge less. You also have processing fee, and other overhead.

Edit: Deleted a few random meaningless chars at the end. My son typed them while I was not watching.

If, for example, Hertz wanted to stop requiring a $250 deposit on vehicles and instead take a $22 fee, they could approach any large insurance company and find out if they would underwrite it. That company's actuaries would assess the risk of a typical Hertz customer and either say that, yes, $22 is enough to cover losses, or no, it needs to be higher.

In fact, Hertz may already do this. A $250 deposit is incredibly low for someone renting a $20-40k vehicle with nothing more than a driver's license and credit card.

Source: I'm a partner in an insurance startup.