DJI pays shipping both ways . Take the Drone to a UPS store & have them pack & Box it , there is a charge .
Its is easy just to get box big enough & just use good packing . They will send it back in a plain box with the egg crate type packaging as the original box it was purchased in .& you have a shipping box if it ever has to go back .
You can send it back in its original box , as long as it is put in another Plain box , not sure you will get you original package box back .
I would not separate the Gimbal from the Drone , let them do all the work .