Error 10081 - Invalid shipping package
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: