I don't use a box at all. I remove everything removable. I secure any sharp edges (Schwinn kickstand, dropouts, top of the seat post, etc.) by wrapping with cardboard & packing tape, tape a foam block between the rear dropouts, label the frame in at least 2 places, then use a manila shipping tag to attach the postage using USPS parcel post. The US Post Office does not require a box, so long as there are no sharp edges. If there are other components to ship, I ship them separately.