Added Discount Details by Dishes

Starting from V6/V6Preview5, discount details by dishes are available. The method GetOrderAppliedDiscounts returns the results of discount applications (AppliedDiscountItem), where for each discount you can get both the total amount of that discount for the entire order (DiscountSum) and the breakdown by order items (DiscountSumByOrderItemId). A general description of discounts and surcharges can be found in a separate article.