Special-purpose stamps: airmail, charity, postage due

Stamps · 7 September 2026

Not every stamp paid ordinary postage. Four families were made for a different job: airmail bought a faster route, semi-postals carried a surcharge for a cause, postage due was applied by the post office when payment was short, and official stamps were issued for government mail and never sold over the counter. Each is catalogued in its own sequence, and each survives in a different way.

Airmail

Airmail stamps paid the surcharge for carriage by air, so their denominations follow air rates rather than surface ones. They are usually easy to recognise: an aircraft in the design, a word in French or the local language meaning air post, and often a distinctive size. Because air rates were expensive, high values are common in this family and genuine commercial use of the highest ones is uncommon – the kind of scarcity discussed in the guide to what makes a stamp rare.

Semi-postal and charity

A semi-postal shows two figures: the postage and a surcharge that went to a cause – disaster relief, sport, health campaigns. Recognise it by the plus sign or the second value printed beside the first. They were sold for limited periods, often bought by collectors as a donation, and consequently survive mint in far greater numbers than used.

Postage due

These were never sold to senders. The office applied them when postage was missing or insufficient, so a used example implies a story: an underpaid letter, a rate change, a mistake. Designs are usually plain – a number and a word – because nobody needed to be attracted to them. Used copies on cover are the interesting state, and they are scarcer than mint ones for the simple reason that most were destroyed with the envelopes.

Official and service stamps

Issued for government or departmental mail, often overprinted from ordinary stock rather than printed separately – the mechanism described in the guide to overprints and surcharges. They were not sold to the public, which makes their postal use a matter of record rather than of collecting demand.

How to recognise them in a mixed lot

• Two numbers on one stamp usually means a semi-postal.
• An aircraft or an air-post inscription means airmail, even when the design is otherwise ordinary.
• A plain numeral design with no country picture often means postage due.
• An overprinted word in a formal typeface may mark an official issue.
• Separate catalogue sequences. These families are numbered apart from ordinary stamps, which is why they seem to vanish when searched among regular issues.

How many there are

The open archive of 80,472 stamp images from 271 countries and territories records 851 airmail items, 409 semi-postals and 256 postage due – together barely two per cent of the whole. That is the point worth taking away: these families are small, distinct and frequently overlooked in mixed lots, which makes them a practical direction for a collection built on attention rather than on budget, as in the guide to collecting on a budget.

Open the catalogue: world stamps, Soviet stamps.