Bag Labels. 
How does a letter get from point A to point B? I don’t mean the routing, but rather the physical process. When you mail a letter in your local post office, it is cancelled and sorted to join other letters going in the same direction. These are placed for protection in a mail bag containing a destination label.
For example, a letter from Dublin to Muleshoe, Texas will join other US bound letters for movement to a US Post Office, probably New York or Boston, then resorted a placed in a bag to Dallas. In Dallas, it is re-sorted and rebagged to Amarillo, and so on to Muleshoe where it is sent out for local delivery. All these bags require a small label to indicate their destination. These are collectable, but seldom seen by the public. The same happens with mail within Ireland.
A related area to this are the Letter Bills or Parcel Bills which determine how money collected is shared between postal authorities. Each shipment of bags is documented, and the accounting system tracks how much went where. These parcel bills are transferred in envelopes marked “Bulletin de Verification” and are sent between countries. Irish covers bear the notation O.E.41. (Figure 4). Ireland may ship the US 1000 bags in a period, while the the reverse flow is 1200 bags, thus Ireland would “owe” the cost of moving 200 bags. Parcels use a similar system. The O.E.41 is used for both replacing the older O.E.37 used for parcels. (Figure 5).
For information on Letter Bills and Parcel Bills, see The Revealer, Fall 2006, “Moving the Mail – The Universal Postal Union and the Bulletin de Verification Registry Envelopes of Ireland”, p 20-25 by James Lawless.