This eBay error can sometimes occur when you have selected to use Australia Post Calculated Postage on your eBay postage policy.
Australia Post Calculated Postage on eBay requires the package weight and dimensions of a product, in order to correctly calculated 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
Note: Unfortunately, for merchants using "Shopify" we are unable to import product dimensions via the Shopify API, so it isn't possible to use calculated postage.
If product weight and dimensions are importing correctly into our platform, the issue may be related to offering International Postage.
Please try removing this first to see if it resolves the issue.
If so, it may be that the International Postage service selected only accepts items of a certain size.
Try selecting a different International Postage service so see if this works.