Don't you have to enter a credit card before it gives you the disc? I imagine they will just charge you later though maybe it'll slio through the cracks.
I doubt it'd slip through the cracks... The late fines are fully automated. The advertised price is a daily rental rate. For every day you have the item, you get charged that amount. Once you hit the maximum amount, it stops charging you and you can keep it (at that point, you've paid the full price of a new movie or game, but just own a second-hand one...)