Shopify does not allow discount codes to be applied to any draft orders, and this unfortunately includes all third-party apps which rely on draft orders to function. This is therefore an obstacle which many third-party apps face when using draft orders - not just Bundle Builder.
To read Shopify's community pages on the limitations of discount codes across all draft order apps,click here.
The Effect on Multiple SKU Bundles
Due to this limitation of the Shopify platform, discount codes therefore cannot be applied to Multiple SKU Bundles.
If you would like to give your customers the option to use a discount code, you must ensure that you are using the Single SKU Bundle type.
To learn more about the differences between Single SKU Bundles and Multiple SKU Bundles, please read our guide on types of bundle.