Best Buy is horrible. Seriously horrible. I have friends who work there and I like them, but otherwise they are a bad joke. I don't believe in karma, but if there is karma it's simply the universe's way of retaliating for their outrageous rip-offs and rude, rude employees (ESPECIALLY management).
I wouldn't feel remotely bad about keeping the system. I don't know if the law is on your side or not. If it was a mail transaction, I'm 98% sure the law is on your side. But since it took place in person, I just don't know.
My worry might be warranty issues, ya know? I'd call Apple and ask them what to do to be honest, just out of fear for my warranty. That's your choice though... just be aware of the risks.
THX, yes, stores budget employee error in their pricing. And best buy has *TONS* of room for that. Of course, they only hire stupid people to work there (no offense to my friends who have worked there), you know - the kind they feel they can brainwash into ripping people off all day every day without giving them a commission to do so; and have them not leave work feeling bad about what they're doing because they don't even know their store is so outrageously overpriced it's unethical.