You know what would be kind of handy for parking payments? Linking to vehicle toll transponders. That way, as long as everyone has a toll account, you wouldn't even need boom gates, just the archways that detect and charge accounts for entering and leaving. You'd still need to impose the same constraints as normal parking payments, such as being dependent on the duration of your stay. The biggest problem I can think of would be waiving the fee for big spenders. The current procedure is to take your receipts and your barcode card to the concierge who makes a note in the database that lets you out for free. Without taking your toll transponder directly to the concierge, you'd either need some other way to identify your vehicle to the database or you'd need a card to show, which brings back the boom gate and ticket reader. Maybe in that case you can do without the boom gate, though.
Mokalus of Borg
PS - And you have to rely on toll transponders.
PPS - Which can sometimes be a bit unreliable.