Hello,
Brian here from the Shopify Guru team. :) I hope the new year is treating you well.
For Amazon to automatically fulfil orders, you must ensure that your shipping rates match up with their own as outlined in our support doc here: https://docs.shopify.com/manual/settings/shipping/fulfillment-services#amazon
If there is a difference between the shipping rate that is applied to the order, and the shipping rates that Amazon recognise, then it will result in this error, so I would suggest double checking your shipping rates and ensuring that they match exactly. The three rates that fulfillment by Amazon requires are :
- Standard Shipping
- Expedited Shipping
- Priority Shipping
This has been something that has caught out a few merchants before, and there is actually a wonderful thread on this, which can be found here: https://ecommerce.shopify.com/c/payments-shipping-fulfilment/t/amazon-fulfillment-by-amazon-fba-shipping-rates-integration-setup-212765
I hope this helps clear things up a bit, and if you have any other issues at all please do not hesitate to get in touch with us. Our gurus are always available to assist at support@shopify.com. :)
Have a lovely day!