This eBay error can sometimes occur when you have selected to use Australia Post Calculated Postage on your eBay postage policy e.g.
Australia Post Calculated Postage on eBay requires the package weight and dimensions of a product in order to correctly calculate the postage cost for an item.
Please check that any products affected by this error have the weight and dimensions in our platform e.g. on the Product Details page you should see the following type of data showing:
Note: Unfortunately, for merchants using Shopify we are unable to import product dimensions via the Shopify API. There is a work around for this that requires the use of Shopify Meta fields. By setting up dimension data through Meta Fields in Shopify (length, width and height values) and mapping them within Omnivore we are then able to send the necessary information that eBay require.
Please see this FAQ here for steps on how to setup Shopify Meta Fields
If product weight and dimensions are imported correctly into our platform then the issue may be related to either offering International Postage, or that your dimensions don't meet the Australia Post minimum weight and dimensions for the service you've selected.
If you offer international post please try removing this first to see if it resolves the issue. If it does it may be that the service selected only accepts items of a certain size. Try selecting a different International Postage service to see if that works.
If that doesn't work, or you only offer domestic post please check the minimum size and weight that Australia Post permits for the service you've selected and ensure that the size and weight you've entered for the products meet those requirements.