[{"OrderId":"406-5309498-5972326","revenueHeader":{"Principal":982.14,"Product Tax":117.86,"Total":1100}},{"OrderId":"506-5234568-5934567","revenueHeader":{"Principal":382.54,"Product Tax":34.46,"Shipping charge":30.5,"Giftwrap charge":27.5,"Total":234}}]

db.collection.aggregate([{$project:{revenueHeader:{$objectToArray:"$revenueHeader"}}},{$unwind:"$revenueHeader"},{$group:{_id:"$revenueHeader.k",total:{$sum:"$revenueHeader.v"}}}])