Instructions
Download the deletable mailbox mod from Leefish.nl (see references for link). When the download finishes, double click the file and extract it to your desktop.
Open "My Documents." Double click on the file "Electronic Arts," then "The Sims 3" and "Downloads." Copy and paste the deletable mailbox mod into the "Downloads" folder.
Launch "The Sims 3." As the game loads, it will install the mod. When you get to the game's main menu, open the file that needs the mailbox replaced.
Navigate to "Buy Mode." Select "Outdoor Items," then "Lawn Ornaments." Scroll through the list of items until you find the mailbox. Click on the item to buy it, then place it on your lot. Save your game. The mailbox will now be replaced.