Here are some common troubleshooting tips:
Make sure that your API keys and Merchant IDs or Client Secret IDs are all entered correctly. It is best to copy and paste this information directly from the Zip Merchant Portal under Add Zip as a payment option.
If you’re using Shopify, make sure test mode is disabled. At this time, it is not possible to test the gateway in this mode. You will need to complete a real order and refund it from your platform.
If you’re using WooCommerce, another plugin might be conflicting with the Zip checkout. Remove any conflicting plugins first before continuing.