This simple mod makes the default mailbox deletable, and buy-able from the catalog, like most objects. This is so you can replace it with custom mailboxes. Its location in catalog is Outdoor/Lawn Ornaments, price 0 simoleons. Install as you would normally install any package file. This mod will only conflict with mods that alter the OBJD of the default mailbox.
I have custom mailboxes available for download HERE and HERE .
If you want to delete your mailbox from buy/build rather than via the debug menu (which can cause glitches in an already glitchy game) then this mod will do it. WITHOUT GLITCHES.