Why does this happen?
This occurs when SWRemote is attempting to submit a specific instance of an item that is no longer seen as valid by SuccessWare21. The item was most likely removed and then added back onto the Invoice and therefore has a new item ID.
Simply remove and re-add the item in question and attempt resubmitting the invoice.
If you experience further issues, please call the SWRemote support line at 1-800-566-6940.