Customer notes when entering invoices

Hi,

How are people handling it when you have customers that need a PO on their invoices when you have many that do not.  How do you recall which one needs PO when requested by their accounts Dept??

Is there anyway of making a note to when entering invoices to show this customer must have a po??

Would love some kind of optional pop up note individual for each customer that does not print on invoice.  Like "must include Stacey when sending invoice" or the like.