Why not just change the letter into something with Moon Duration? The recipient just needs to wait until the duration expires, at worst.
If you feel this ain't long enough, first, read this:
=> you could design a Ring/Circle spell of the Type I persuasion (affects what is inside the ring at the time of casting, even if it leaves the ring, so long as it is intact) that changes your letter into, say, a lump of wood.
Then, you need, say, a small wooden circle.
Put the letter into the circle, cast the spell.
You just send the lump of wood, and then, either, after a while, you break the circle, or your correspondant just casts some small Perdo Vim effect to cancel it, turning the lump of wood back into a letter