The Cat Who Played Post Office is the 6th in the Cat Who... series by Lilian Jackson Braun. It has dropped to $1.99 at Kindle and Kobo US.
Kindle US:
https://www.amazon.com/Cat-Played-Po...dp/B000OCXJ7U/
Kobo US:
https://www.kobo.com/us/en/ebook/the...-post-office-1