I used this stamp, as I had some laying around my desk.

I was under the impression that I would not need to pay anything extra with this stamp attached, but when at the Post Office, the lady only assigned a value of £1.45 to the stamp, meaning I needed to pay an additional 90p for the 1st Class Signed For service.
It's not the end of the world by any means, but just for curiosity (and for future posting), I wanted to know if I've been mistakenly overcharged by the Post Office, or was everything done correctly?
Many thanks!
Here is my receipt confirming this.
