Discounts work with Zapiet - Rates by Zip Code out of the box. This applies to any discount on the order, whether it's a Shopify discount code, an automatic Shopify discount, or a discount created by a third-party app.
The rate your customers see at checkout depends on which rates version your store is using, which you can check in our Rates functionality article.
Discounts only affect price-based rates. Weight-based rates are matched on the weight of the order, so a discount code never changes which rate is returned.
Shopify Function rates
Rates are calculated after any discounts have been applied. If a customer enters a discount code that lowers their order total, the app matches the rate against the discounted total, so the rate shown at checkout reflects what the customer is actually paying.
Rates with carrier-calculated shipping
Rates are calculated against the order total before any discount is applied.
If a customer enters a discount code, the app still matches the rate against the original order total. The rate shown at checkout is the one their pre-discount total qualifies for.
The post-discount cart total is not available through the Shopify Carrier Service API [Shopify limitation].
Example
A store has a price-based rate offering free shipping on orders over £90. A customer with a £100 cart applies a 20% discount code, bringing their total to £80.
With Shopify Function rates, the app sees £80 and charges for shipping.
With rates with carrier-calculated shipping, the app sees £100 and gives free shipping.
If you're seeing a different rate than you expect after a discount is applied, let us know the details on chat and we'll be happy to help.