A client since 2004 recently underpaid $5 each on two invoices.  Sadly, our pricing has never changed for this client.  After e-mailing them about it we received this response: 

"I sent the fees to my vendor manager. He says we will not be able to change the fees till the end of the month. This gives us two choices. We can wait to send orders till next month, or keep sending orders at the old pricing and then update it at the end of the month. Please let me know which you prefer."

I am going to have to hold my response until I regain my composure.