[adam-fn]

I had a similar experience with discounted-if-paid-promptly hospital bill. I got a call saying, essentially "we didn't move the money around our internal systems in time and now we want the rest of the money".

Since I paid with a credit card I could make the case that I was charged before their deadline and they let it go.

[robertcope]

I've had similar things happen with my electric bill. I noticed at some point that I had been paying small late fees. Looking into it, they would cash my checks, but not credit the account until the day after the payment was due. They stopped doing it after I pointed it out, but that took a lot of convincing. I'm not convinced that it was an accident.