The passage on page 73 on Oskar's ideas on placing signs on ambulances saying whethere the person inside is "going to be alright" or whether he/she needs to be saying "I love you, goodbye". This would be a way for someone dying to say a final goodbye, which is probably symbolic for Oskar's feelings about not getting one from his father.
I think that the reason that this is included was possibly because the author is trying to express how Oskar wishes that he had known that he needed to worry, despite all of the messages telling him not too. Also, by having the message read "GOODBYE! I LOVE YOU! GOODBYE! I LOVE YOU!"(pg73) shows that he wishes that he had a chance to say goodbye to his father.
Here Oskar is inventing stuff and starts to talk about ambulances "What about incredivly long ambulances that connected every building to a hospital?" (258)
His sign for ambulances is another of his many Inventions.
Oskar has a very ambigious way of telling people his emotions, but when he refers to the ambulances, one can see that he is truly scared of death, and uses the examples of ambulances to show why one shouldn't worry.
Pockets refers to big pockets because both are ways to keep from losing the ones you love and knowing wether or not they are ok.