This may be a little more work, but you could have just one location and invoice the shipments through Order Entry from your ship from location and have all the invoice start with a "S" and then the regular invoice number that the system would produce.. When they use the goods, you credit back the invoice and re-issue it from the regular location using the old invoice number less the "S". The ship from location would always be negative and you could use a report writer to see is at each client's site.
The down side is having the extra invoices and credit notes on the client's file or set up a separate client to handle the shipment invoice and credit note.