You can add a discount code for a product in your eCommerce store by following these steps:
  1. Navigate to Fundraising, eCommerce, Products tab
  2. Edit a product
  3. Go to Step 11. Discounts
  4. Create a New Discount or add one that is already created
  5. Click Save and Next
  6. Go to Step 18. Publish
  7. Click Publish Product
Your product now has a discount code that can be used when purchasing. Here is further documentation on managing eCommerce - eCommerce Management Guide.