The post box ate mummy's letter :(

Today is mummy and daddy's last day of holidays - back to work for both of them tomorrow, and I'm off to grandmas for a sleepover.
We had a lazy morning and then I fell asleep in my cheese on toast at lunchtime! (See additional picture)
After my nap, me and mummy went shopping whilst daddy did some tidying and hoovering.
Mummy forgot to get cat food and a stamp for a letter she needed to post, so when we got home we had to go for a walk to the corner shop.
I carried mummy's letter so that we didn't forget to post it, and was quite excited when mummy let me help push it into the post box.
But I was expecting it to come back out.
And it didn't.
I was not impressed, and mummy had to try to reason with me to get me to carry on walking home (much to the amusement of the old lady walking her dog!)