I have some clients that will issue an open PO number to bill against until it runs out of money.  They might allocate 25 hours of billable time per PO.  They give me the PO number, and I'm supposed to include it on each invoice until I get a new number from them.

I can put this in my customer notes, but I need to remember to copy it from those notes to the invoice when I send it.  Is there a way to track this better?  I basically get an e-mail whenever that new number comes out, but I am likely not invoicing against it for a couple of weeks or maybe even a month or two.