Most suppliers on Northbound require a credit card to cover the insurance deposit.
(*Please note that debit card or cash deposit are not accepted for this).
Some of the suppliers only take an imprint of the card when you arrive and don't actually withhold any amount from the card but others do.
Once you return the vehicle you'll get that money returned if there are no issues with the vehicle.
If you select a vehicle you can click the Important Information tab to see more info about that supplier's policies.