Well, in your situation you only have a few options. You can't use USB to connect your printer and not expect issues when it has been shut off. You need to realize that for you to figure out a cheap solution you have cost them several man hours which is actually costing them more. If these solutions don't work for you then unfortunately, this is all I have.
1. Try using the scripts to map and unmap printers.
2. Buy a networked printer. It can be plugged directly into the router in the warehouse. This will cost about $150 from Office Depot. Probably cheaper if you shop around.
3. Map a shared drive that you call Invoices and create a distribution group (DG). This get's rid of the need to train to use one mailbox - everyone get's the email. The office can put the file on the share and shoot an email to the DG.
4. Create a DG and have the office email the file. This solution will put strain on the mail server with all the files.
5. Create a scheduled task using the shutdown command. Running this at say 6pm or 7pm will not only allow environmental consciousness but also leave the workstations on. Look here for info on shut down command http://www.computerhope.com/shutdown.htm