The scooter, with the rear basket attached, is oversized and won't fit in the sizing box. Some drivers, some buses will let it slide. It's not just buses. You might have to remove the basket for some boats. The scooter, with the basket, can be a tight fit on some elevators.
It comes off easy. Probably not an issue if you have a companion. My memory is you loosed two nuts and it slides off. Piece of cake if the basket is empty. Can be an issue if you have the basket filled with heavy items like bottled water.
Last trip Gold Mobility ran out of baskets. We could have gotten one delivered but timing didn't work. It was easier just putting a back pack on the back of the scooter. Plenty of room for a few bottles of water, rain ponchos, snack food etc..