Quick answer: Discounts combine only when Shopify’s combination rules and eligibility conditions allow them. Product, order, and shipping discount classes must be compatible, and some discount types still compete for the same items or customer/cart conditions.
The real symptom
- A code applies alone but fails when another discount is active.
- Automatic discount and discount code do not stack.
- Checkout says the code could not be used with existing discounts.
- A discount app generates codes that ignore the expected combination settings.
Likely causes
- Combination settings were not enabled on both relevant discounts.
- The discount classes conflict: product, order, or shipping.
- Both discounts target the same items in a way Shopify will not stack.
- Eligibility, market, customer segment, minimum spend, or usage limits fail after the first discount is applied.
- Scripts, Functions, or app discounts override expected native behavior.
Exact admin path
- Go to Discounts.
- Open the first discount.
- Review Combinations and enable the classes it may combine with.
- Open the second discount and verify the matching combination setting.
- Check customer, market, product, collection, and minimum-purchase eligibility.
- Test with the exact cart, customer, market, and code sequence.
Simple checks
- Inspect whether the discount is product, order, or shipping class.
- Test each discount alone first.
- Temporarily disable app discounts or Functions when troubleshooting native combinations.
- Check checkout messaging for the specific rejection reason.
Do not do this
Do not promise “stackable” discounts until you test the exact cart scenario. Combination settings are necessary, but they are not the only rule.