I regularly use these methods and both work fine. Notice that they work only with the original expansion or store worlds, not with custom versions of the worlds.
If you want to have a different world’s mailbox and trash can on an empty lot, create a save with the world where you can find the mailbox and trash can. Choose an empty residential lot, or create one in edit town. Go to build mode, and add something so the lot won’t be totally empty : a wall, a tree, whatever. This will allow you to save the lot to your library so you can export the lot to another world.
If you already have a house and want to change its mailbox and trash can, you absolutely can do it too. Select the lot you want and place a copy of it in the same world. If the lot copy is inhabited, throw its inhabitants out. Change the lot type from residential to community, which will automatically delete the mailbox and trash can. Save this community lot to your library. Start a save with the world that has the mailboxes and trash cans you want. Go to edit town. Place the community lot in this world. Change its type to residential. A new “local style” mailbox and a new trash can will be automatically generated. Save this residential lot to your library.