Shocking attitude and poor ownership

Had a call from a very rude woman at their accounts department, telling me I owe them for cytology tests that they hadn't previously invoiced me for. After questioning why it had taken so long, I was quickly told I was being difficult, and asked if I am going to pay today or not. It then became an exchange of her answering my questions with further questions. The back story is that they didn't have sufficient material to analyze so it had to be re-done for our terminally ill dog (which was confirmed after the second set of tests). I was only ever invoiced for these, and received a zero balance statement once settled. On top of this, they made blatant spelling errors with the names on the invoice, with little acknowledgement nor apology, which is not something we need, given our situation.