This book is sold as a comedy novel and for the first half of the book I would say that it fits that description well enough. For the second half of the book, it's a novel: the comedy and light relief fall away. I'm not sure all of this is a bad thing since the comedy aspect of the first half didn't dominate and neither did it intrude. I suppose the basic underplot of saving a town's Post Office from International Wheeler Dealerdom was funny in itself!
I liked the way this book and the main characters all developed and the storyline was credible and moved along very well.
My lasting impression of this book won't be the way the post office was taken over by a not very nice chappie; but that Palin himself seems to sit four square as the main character. The knowledge of Hemingway and his works that was liberally spread around this book was real knowledge taken from someone who has clearly read, learned and appreciated everything that Hemingway had to offer. I even have the feeling that Palin owns THAT chair: the chair that is at least partly central to the plot.
Drop the comedy tag and approach this as a good read and you'll enjoy it I'm sure.