Yes, you just download it from the website once the item sells and then you can drop it off at the post office you typically go to.
They usually take 1 day to return my emails. I agree, state very clearly that you checked the bag had no odor before it was sent, and that it it now has a scent it was due to the buyer causing the scent. If you packed it in a lot of plastic wrap, that can add a fairly strong scent, but it would air out in ~24 hours or so. I wouldn't mention this though in your email to Tradesy, best to keep it short and definitive.