What is the setting of the novel The Mailbox?

There are two books you could be referring to, coincidentally both set in the South.
The Mailbox by Audrey Shafer is a mystery set in Virginia. After the death of his uncle, a boy decides to keep the situation secret to avoid the foster care system. The mailbox portion comes into play as he receives mystery notes that threaten his situation even more after his uncle's body disappears.
The Mailbox by Marybeth Whalen is set on the coast of contemporary North Carolina, specifically a place called Sunset Beach. The mailbox here is a place for the main character to deposit her feelings and letters throughout her life. It is my understanding that this revolves around a real landmark out there on the beach. This book is the first in a series of modern romance novels.
https://www.goodreads.com/book/show/145310.The_Mailbox

https://www.goodreads.com/book/show/7443281-the-mailbox