With the Shop app, you have the ability to offer promotional codes that provide discounts on paid products to visitors in a Store. When a promo code is used, you can also details saved to Salesforce.
In order to successfully save the promo code to Salesforce as part of the OLI Temp and Opportunity Product records, you'll need to make sure both objects have this field. When defining the API Name, enter only the string proceeding __c. Salesforce automatically adds __c to custom fields:
Field Label: Product Discount Code
API Name: sbx_ProductDiscountCode__c
Data Type: Text (255)