Using a post-hooked cloud script to override product purchase rewards

In the case that you want to double the rewards of a promotional item, instead of offering the same items for less $, you may want to use PurchaseRewardHook feature to override the configured reward amounts for product purchases.

The following example will show you the steps to complete this process.

First, create a hook script as below. it's well-described as the in-line comments. It is going to override the rewards for a purchase given promotion status.

"use strict";
function main() {
	var response = {};
    // The goal of this routine is to adjust the purchase rewards if a promotion
    // is in progress - according to the custom JSON in the promotion record.   
    // First check to see if promotions are active
    if ( data.promotions.length > 0 ) {    
        // Then check to see if this item will actually be purchased...
        // (Not much point in doing the calculations otherwise)
        if ( data.alreadyPurchased === false ) {         
            // Okay - lets see what item is being purchased
            var itemId = data.itemId;
            var newRewardData = {};
            var newPromotionId = data.promotionId; // no idea, may change this based on promotionalOverridesApplied below?
            var currentPromotionPriority = -1;           
            // Iterate through the promotions to see if there is one with replacement currency values
            // Note that we've included a "priority" field in the customJson portion of the promotion,
            // so that this script can tell which priority rewards to apply (in cases where multiple promotions are active)
            var promotionalOverridesApplied = false;
            for (var i = 0; i < data.promotions.length; i++ ) {              
                // We don't need to apply the pricing changes - brainCloud will have already done that.
                // We just focus on the custom rewards and custom JSON             
                // First check to see if it's higher priority than the current promotion (if any)
                if ( data.promotions[i].customJson.priority > currentPromotionPriority ) {                 
                    // Now see if there is actually an item that matches in the promotion
                    if (data.promotions[i].customJson.itemIds.hasOwnProperty(itemId) === true ) {                      
                        // Booya - let's grab the updated currency and customJson values
                        currentPromotionPriority = data.promotions[i].customJson.priority;
                        newRewardData.currency = data.promotions[i].customJson.itemIds[itemId].currency;
                        newRewardData.extra = data.promotions[i].customJson.itemIds[itemId].extra;
                        //newPromotionId = -1; // ONLY IF want data.promotionId removed from transaction
                        promotionalOverridesApplied = true;
                        newPromotionId = data.promotions[i].promotionId;                        
                    }                     
                }               
            }            
            if ( promotionalOverridesApplied === true ) {
                response.rewards = newRewardData;
                response.promotionId = newPromotionId;           
                return response;             
            } else {             
                return null;              
            }             
        } else {
            return null;
        }     
    } else {
	    return null;
    }
}
main();

Then, create a promotion for your app from the portal promotions section. Make sure to set the custom JSON fields accordingly to the script above.

Note: replace the rewards rule as you need.

{
  "priority": 100,
  "itemIds": {
    "barBundle1Imp": {
      "currency": {
        "bars": 12
      },
      "extra": {
        "specialItemReward": "bonusItemA"
      }
    },
    "gemcollection": {
      "currency": {
        "gems": 7
      },
      "extra": {
        "specialItemReward": "bonusItemA"
      }
    }
  }
}

After then, go to Design | Cloud Code | API Hooks page, click Create, hook the script you just created as post-hook to PurchaseRewardHook operation as below:
Done!

Pro-tips:

You can verify the rewards changes by calling GetEligiblePromotions() or RefreshPromotions() from API Explorer for a user who is eligible for the promotion you set. (Note that the rewards for bars is overridden from original 10 to 12, and the gems is changed from 5 to 7)
The original rewards for bars:
The original rewards for gems:
After the above script is hooked:
Once the purchase is finished, you can also verify the rewards from the Transactions page as well by viewing the Rewards Data of that transaction entry.