Error:
Error 10081 - Invalid shipping package
Resolution:
This error is caused because the Shipping Package Type is not valid for one or more eBay Shipping Service(s) selected.
When this error is returned, you will have to choose a new "Package" value, or change the Shipping Service(s) offered. You can either set the default value for this under Settings > Channels > eBay Main > Shipping Or change it in the single product editor as shown in the screenshot below and then relist the item on eBay.
You can set a default eBay Shipping Package Type in Settings > Channels > eBay Main > Shipping > Shipping Type - as indicated in the screenshot below: