How will the mail become less anonymous by adding a random unique number to it? It's not like you need an ID to buy stamps.
If you buy stamps from a shop, perhaps a register is kept of which stamps were given to which shops.
I don't know how likely any of this is, but it can definitely make stamp purchases more traceable than without the barcodes.
In the coming cashless society, you won't be able to buy anything whatsoever without it being tied to your ID.