This was proposed and discussed during development - and is relatively simply to do - but I removed it from the early development plan. My main concern was trading or sending items is normally an atomic operation, ie it leaves my possession and arrives in your possession in a single action. I didn’t want us to risk player’s hard earned items with a new system until we’d established that it was robust and stable.
So sending items via the Letter box is on the todo list once we’re confident the system is solid.