Postage Due stamps were first issued in 1879 for accountability purposes. Postmasters had to account for their stamp stock: stamps in inventory plus receipts (mostly cash and checks) must equal the total value of the stamps originally ordered. Postage dues operated the same way. They used a postage due stamp to account for the cash they took in for payment of postage on short paid items.

Postage due stamps were used for just over 100 years. The last postage due stamps went to press in November 1985. Two changes brought about the demise of postage due stamps. The USPS required prepayment of postage in full. Thus, most mail was returned to sender for proper postage. And when mail was forwarded to the recipient, they moved to using rubber stamps and other auxiliary markings to track the postage due.

Postage due stamps are considered back of the book material. In my experience, collectors save the front of the book material (with normal Scott numbers) and many collectors save airmail issues too. But when it comes to Special Delivery, Parcel Post, Postage Due, and other back of the book material, demand is lower because there aren't as many collectors interested in these areas.

Postage dues could be an area for important finds. Because fewer collectors save postage due issues and because the USPOD issued them as needed, it's possible that new finds can be made in this area. Postage dues don't have First Day of Issue dates like postage stamps. They were issued without fanfare.

Identifying the postage due issues is not too difficult. A perforation gauge and watermark fluid are all the tools you need

Postage due issues have 5 basic designs as noted by the Scott catalog. Here is the breakdown of the designs and how to identify each one.

Design D1 is the large numeral issues. They are the same size as the large Banknote issues (Scott #134 to #218). They saw use for about 16 years until replaced by a new design in 1894. Identification of these issues is by color.

Most collectors won't have the #J8-14 issues which were a special printing. They are rare. The J1-J7 issues are in a brown ink, showing no red color. J15-21 has a strong brown color, but the color is somewhat reddish. J22-28 is a deep red color with no brown to it.

Design D2 takes the most effort to identify. These issues have a numeral in the center of the design with a ribbon containing the denomination spelled out in letters at the bottom. Identifying these issues requires a perforation gauge and watermark fluid.

J59 and J60 are scarce. J58 is rare, but otherwise, these issues are inexpensive or moderately priced.

In 1930, designs D3 and D4 were issued. They have the numeral in the middle but also have smaller numerals in each corner of the bottom of the stamp design. A perforation gauge is all you need to identify these two varieties

All of these issues are inexpensive or moderately priced.

In 1959, the USPOD switched to design D5, the last postage due issues before they became obsolete. All of these issues are design different and require no further identification. But there are a couple of things to note about this last series.

All of the stamps have the same red printing for the background. The BEP printed the denomination in black ink. In a way, the black ink was an overprint since it's not part of the stamp design. The black ink was printed in the same way that the BEP precancelled postage stamps at that time.

These issues also saw a change in gum. Originally printed with shiny gum, the USPS moved to a dry gum. Most issues are known with dry gum, however the 6¢ is scarce and the 7¢ is rare. These two issues are worth large sums of money with dry gum.

One last point. The 1/2¢ issue, Scott #J88 is not known in commercially used condition on cover. There are a few philatelic covers created for this stamp. If anyone ever finds a commercially used #J88 tied on cover, it would probably fetch thousands of dollars.

Postage dues are relatively easy to identify and outside of a few issues, very affordable. They should be worth your attention.