An arm and a plate are going to run you around $70 plus you will need to take the Mavic apart to get the gimbal out to replace the plate and do some soldering on the arm. If you need to use it in the near future, you dont really have a choice and self-repair is the way to go instead of waiting 4-6 weeks for DJI to get it back to you, and paying them $79 to get another one.
If you are worried about DJI Refresh being voided because you did self repair, you can always get drone insurance through State Farm or other done insurance carrier. Its a better deal than Refresh, most times $60 a year with no deductible, and you dont have to wait an eternity to get satisfaction. Plus you dont have to worry about getting a Refresh replacement which will probably be a refurbished drone.