I totally agree with Inspired by Reggio here. You might even want to shoot off a text that says you mis-typed and of COURSE she will be getting her tax receipt next year as required, but additional receipts cannot be issued at this time. Since she has in writing that you plan not to issue receipts, you could be in trouble if she decides to be even MORE of a cow about it. Best to cover your butt.
Sorry you are going through this. I just had a rough termination last week myself, and I don't wish these types of people on anyone!