I still have the box it shipped in (I'll dig it up shortly, it's at my parent's house).
I think it did mention it was an RMA on the outside of the box. I'll check if there aren't any papers attached to the outside (usually in a plastic insert).
It shouldn't be difficult to prove. The hardisk came from a major hardisk company (Seagate) and they even have it on record that it was an RMA. All they'd have to do is call their line and pull up my case file. I'm guess PBB probably won't care if they're anything like UPS...they'll just insist I owe them money for some obscure reason (i.e. "We verified you don't in fact owe brokage, duties, GST or PST. Now you must pay us for the service of informing you this fact--that'll be $20 please").
I'm still planning to fight it, out of principle. Just gotta be persistent (now if only I could get to a phone to speak with them when they're open. arrrgh).