It’s worth a shot…
ETA: And are people serious about calling this guy an idiot because he misunderstood one thing?
Perhaps, since descriptive linguistics includes the misuse of human language. Many more examples would be needed before any textbooks are rewritten; can you help research? 
For me, a more interesting question would be: Was such an error equally common during the Age of Literacy?
Here’s a weird case in another language. In Thai adjectives generally follow noun. Replacing Thai syllables with the corresponding English equivalent, colloquial Thai has the following oddity:
[ul]
[li] three thousand = 3000[/li][li] two thousand = 2000[/li][li] one thousand = 1000[/li][li] thousand = 1000[/li][li] thousand three = 1300[/li][li] thousand two = 1200[/li][li] thousand one = 1000 (ambiguous, but 1000 rather than 1100 is the usual meaning)[/li][/ul]
A set of three arrows was called a pair of arrows, though the OED lists it as “Now chiefly hist.” One of the quotes (from 1801) says
Probably unfair. He may have just been having a passing idiotic moment. Happens to all of us.
To be a little more precise, it wasn’t just passing. It lasted at least from the moment I called in the order to several weeks later, when I picked it up.
Okay, I’ll agree that never considering the possibility over the course of a month took an extra scoop of specialness.
Giving the benefit of the doubt, if the items were not sold as a set, perhaps he thought you were asking for “two paired” braces which I would take to mean one left and one right.
I think the salesperson was just thick.
It wasn’t a salesman. It was my podiatrist’s assistant.
And I’m still thinking here that there’s some linguistic issue. I’m not ready tomconclude that he was “just thick.”
Seems like you have all the answers. Good luck.
Wait, I’m suspending judgement in the face of your bald conclusion about the intelligence of a person you’ve never met and I’m the one with “all the answers”? How odd.
But, you see, you asked this question on an Internet message board where it is unlikely anyone will meet this person. If that is the governing factor all answers here will have the same validity. And if you don’t want answers from persons who have “never met” this person, this is indeed an odd place to ask.
So, you’re unable to address a question of language usage without reaching conclusions about a stranger’s intelligence?
I’m rapidly reaching a conclusion about a strangers intelligence based on this thread.
Acsenray, Cicero, both of you back away from the keyboard, please. If you need to impugn each other, take it to the Pit – if you don’t, dial it back.
Thanks,
twickster, MPSIMS moderator
FWIW, I think the guy was a bit of an imbecile, too, or just had a brain fart. 
The month thing doesn’t really surprise me. If he got it into his head at first that you just wanted two ankle [del]bracelets[/del] braces then I see no reason for him ever to address it again.