We've taken ours on flights as checked luggage, we didn't wrap it, just put a rubber band around the loose straps and went from there. No dramas at all.
same here. We used to use a big plastic bag that had once been provided by Qantas, but once it got ripped, we just put the car seats through sans bag.
We have the seats already and the cost for hiring them with the car is about $300. They are free to go in the luggage allowance with the kids tickets as they are both under 12 yrs and will be taking a stroller too.
this was also the reason why we generally take our own. For more than a weekend, it's hideously expensive to hire 2 car seats. Plus, DD1 can only fit in a Hi-Liner now (very tall) and most car hire places just use standard booster seats, so they don't fit well. Car seats have always been free for us.