They (credit union) actually gave us a payoff statement with a scheduled payoff date of the 30th. We thought it was strange when the money remained in the account after Thursday. So we went in today and asked if we needed to write them a check or something. The teller showed us to paper we signed to do the transfer on the 30th. Then she called up the real estate loan department and asked: "What is going on?"
They made some sort of lame excuse, which boils down to: "We forgot."
So they backdated the payoff to the 30th, and did an internal account transfer this afternoon. We did not have to pay four more days of interest.
Which brings us to why they don't want to take your check when you just walk in. The check will take a few days to clear and they don't want to eat those interest per diem charges. If you have the funds in an account of the bank holding the mortgage, and the full amount is available, they can generally transfer it the same day. Otherwise they want a wire transfer for $30. (Although ACH takes overnight between banks.)
I was tempted to use the credit union's Bill Pay web page. We used it for the mortgage payment every month but we could just pay extra if we wanted. So what happens if you Bill Pay the principle balance?