I don't know about the hold, I don't think they put any hold as I had a very low limit left on my card on it was ok last time I rented,
but a hold does not mean they take money from you. A hold is deducted from the maximum amount you can use with your card. You don't need to have that money on your account
you may only get charged when you buy extra
such as child seat, satnav or any upgrade.
you'll also get charged for the full tank of gas if you go for the "return fuel at any level" option
They only swipe the card at the desk, you'll then get an invoice with the estimated charge left to be paid
when you return your car, you will be issued a receipt (inside the garage, no need to go back to the desk, an employee will have a hand held machine to finish the transaction
this receipt will show the final charge (most probably the same amount as the estimated charge at pickup)
payment will be processed in the next 48 hours or so after you return your car