The Royal Philatelic Society London has published yet another book on Rhodesia and Nyasaland Mails, once again written by the same distinguished team of experts: Brian Trotter RDP, RDPSA, Hon. FRPSL, Patrick Flanagan RDPSA, FRPSL, and Keith Harrop FRPSL.
As stated on the RPSL website:
One of the less-trodden collecting paths in postal history is that of the postage due mails, which is a particular interest of the authors, and is therefore given centre stage with a relatively in-depth review of the subject regarding the Rhodesias and Nyasaland postage due mails. Perhaps a slightly more descriptive title for this first section of the book may be “More to Pay Mails”, as there are several reasons why there could be “more to pay”.
Postage due mail normally refers to unpaid or insufficiently paid mail, which is the most common form of postage due mail. However, “more to pay” can also be required when invalid stamps were used to prepay postage, or if a registered piece of mail was not correctly registered at the post office counter or an item was sent without registration for which registration was compulsory. These resulted in an additional charge and were referred to as mail “Posted Out Of Course” (POOC). These, along with other types of postal material, also fall into the broad “more to pay” mail category.
More details and how-to-order instructions:
https://www.rpsl.org.uk/language/en-US/Home/Publications/Books/Rhodesia-and-Nyasaland-Mails-Postage-Dues