I had written to Charles and Camilla after the Birth of Princess Charlotte and received this letter in reply. You can tell this is a form response by the generic message, handwritten date, and absence of my printed name on either the top or bottom of the page. What I really liked about this message, that you didn't see on the Queen's, was the mention of how thrilled Prince Charles and Camilla were at the birth, the reciprocal best wishes they send in reply and the fact that I actually know who this letter was written by. Even though this is a form letter it is much more personable.
The letter was written on a half sheet of paper. The writer, Miss Claudia Spens, has written all of my responses from Charles and Camilla, except of the Hand Signed Card from Camilla. Although this letter came 2 months after Princess Charlotte's birth it still falls in the average/expected response time for British Royals.