I am flying United Airlines from Shanghai to St. Louis via EWR. While living in Shanghai I purchased a digital piano and am wondering if I will be allowed to check it in it's original shipping box. It is oversized but not beyond the 115" oversize limit and should fall below the 70lb weight limit of business class travelers. I tried calling United and could not get a clear answer from them as to whether or not I would be allowed to check it in its original box. I know this probably isn't as safe as a travel case but I do not have one and don't think I can get one in time, plus it will be in all the styrofoam that it was originally shipped in, so I think it will be fine. Anyone have any similar experience?
Yes, you can check a cardboard box as you could a piece of luggage.
Keep in mind, though, that the handling a piece of luggage will receive before and after a flight may be considerably rougher than was envisioned when the manufacturer designed the packaging. In particular, the box may have been intended to be packaged with others in a larger and sturdier crate, and may not be ready for the jabs and scrapes it can receive in normal luggage handling operations. And if it does break, you will be entirely responsible: checked luggage damage compensation does not apply to electronics. So, go for it if you like, but beware.
Willeke points out in comments that there are things you can do to reduce the risk of damage. At the very least, put packaging tape over every openable edge of the box, to reduce the risk that it'll pop open. You can also wrap the box in brown paper and tape that, which will reduce the likelihood of scrapes. Alternatively, many airports will have a kiosk with a person who will wrap your luggage (or box!) in absurd amounts of cling film, which should have a similar effect. The gold standard, though, would be to pack the box in a slightly larger box, with packing peanuts around it. Your local packaging store will be able to do this for you for a reasonable fee, even if you're not using them to actually ship the box. If you know what the oversize limit is, the store can make sure they don't exceed that.