I don't have an answer for you, but why can't you remove it from the box? The policy is here: https://www.united.com/web/en-US/con...heelchair.aspx

It it, it states that excess/oversized fees may apply for "may apply for checking additional wheelchair(s) that are used for recreational purposes." While your boxed chair is not for recreational purposes, in the box, it really isn't being used for mobility/assistive purposes either, which is the stated intent of the waiver.

Again, I can't give a definitive answer, but it would seem out of the box, maybe in a bag if protection is what you are looking for would have you covered, as you aren't IMHO, using it as an assistive device (waived) or as a recreational device (charged).

Possibly reservations can find more detailed answers when you are looking to ship a wheelchair as cargo, but this is the best I could find.