Print out the delivery note or invoice and stick it to the items and move them to a dedicated area in the warehouse, then you can physically see what is shipped/invoiced and still in your warehouse. As far as reports go you can write a report for shipped not invoiced - but not for invoiced not collected. There is no status in the system for invoiced not collected. So maybe ship the items but only invoice when the customer collects, then you can track that on a report.