FWIW, I brought a project bike along as luggage on a flight from Canada to the US last year. The bike was completely dismantled and packed in a rectangular box, about the smallest it could fit into with padding. I flew United. The agent at the counter wanted to know what was in the box. I said a bike. She said we charge $100 for bikes. I said, what if it were bike parts? She said, then it would be $100 because of the weight (~40lb/20kg). I grew a little suspicious at that point, but getting up in arms on an international flight into the US didn't seem like a good idea, so I just paid the price & let it go.