The Number 17 having been discussed beyond my ability to take it all in, I would like to add that I think that Ennis having a mailbox is meaningful in that:
1. He has given up the P.O. Box necessitated by frequest moves, it was a place where Jack could reach him.
Ehm, Jack was dead when Ennis put up the PO box
3. It was symbolic in that he was acknowledging that he was going to be right there, was not going anywhere else, and that the only opportunity he may ever have to live elsewhere was gone.
What a sad, but logical view. I can't help seeing the exact same action in a more positive light, somehow like making a declaration : Here I am, Ennis Del Mar.
Now I never understood why he was in Higgins Gift Shop. He never struck me as the kind of person who would go into a gift shop.
Depends on what associativities you have with the term gift shop. Like others have already stated, I thought of a small, old-fashioned corner-shop.
What I found interesting is the fact that Linda Higgings called Ennis by his first name. She knew who he was and they were on friendly enough terms that she called him Ennis.
I somehow picture Linda Higgins as a motherly, middle-aged woman who was on friendly terms with all townsfolk who regularly stopped by her shop.
When Jack came for his surpise visit after the divorce, he had to ask ten different people to find Ennis. But Linda Higgins knew who Ennis was. I like her for this.