Thankfully, while I'm not exactly entitled to use FedEx rates, my uncle is head of security so I can always have him fax over documentation saying otherwise.

That being said, I've never been asked to provide any proof. It's not something I just do all the time, but I've never had any problems when I have.

I'd say your best bet is to just go in being really friendly from the start, and then if they ask just make pretend like you left it at home.
If you're friendly enough, they probably won't care too much, unless they're seriously ovebooked. They'd rather rent it out to you at a discount than have it sit empty, unless they think you're just going to be a pain in the ... in general.