Shipping cost information incomplete
<ShortMessage>Shipping cost information incomplete.<ShortMessage>
<LongMessage>You did not provide a value for additional shipping cost. The same value you provided for shipping cost has been copied to the additional shipping cost field.<LongMessage>
Like with any other warning, it would not prevent you from listing an item successfully on eBay or from updating an existing listing and they are typically returned only to notify you, that there might be something you would like to double check and as outlined, there aren't any specific values for the Additional Cost field in the Shipping section of the product editor, which would typically regulate whether there would be other and different shipping charges for the additional quantity of one item, that the buyers can purchase and eBay would apply the same shipping charges to each additional item.