Here are my collected thoughts on the topic.
Firstly, I like the book, it is fun and the heritage copy at Grandma's provides my small son with much jollity.
It seems to me that the theme of the book is customer rights and discernment, both good pursuits.
Why did the letter-writer correspond with a zoo in the first place? "I wrote to the animal shelter to send me a pet. They did. The End."
What was in that letter? Was 'pet' code for: "I have acres of space and suitable containment facilities for elephants/giraffes etc. Please send me a creature from a safari environment"?
Maybe write to a different zoo and see if they are a bit more on the ball, rather than doggedly (ho ho) persist with the original establishment?
I feel like the zoo was probably owned by a less assertive individual - you may know one of these kind-hearted people who seem to accumulate pets. "I wrote to the zoo to send me a pet. They sent me an elephant. I didn't have the heart to tell them it was totally unsuitable so I tried again. Now I have a giraffe, a monkey, a snake etc. Ah, man, they are all cute in their own way but the petfood shop takes ages."
It could also be a stern reminder to keep your receipts.
Perhaps this seminal text predicted the comic inappropriateness of item subsitution from online supermarket shopping. "We see that you ordered a pet. We didn't have the requisite puppy so we upgraded you to a camel." Er, thanks...
Anyway, that is what happens when I deploy my English degree training at every opportunity, no matter how irrelevant. It is also why I love children's literature because it celebrates the absurd, and rises above such inspection. Hopefully more stories like this one which is simple yet timeless will emerge from the workshop!