A mystery on a Canadian postcard

The GPU/UPU rules only applied to mail between or through members. The domestic policies and tariffs of members were never dictated by the treaty or the convention. ...

The country of origin9 keeps all prepaid postage. This was constructed on the assumption that every letter sent, receives a reply - there is a natural equilibrium such that the bookkeeping is not necessary. And because of this equilibrium, a letter that is prepaid, is free of charges at the destination.

Franking (Prepayment in Evidence): Franking must be in evidence in the form of stamps or printed postal stationery. Franking which is not readily visible is to be interpreted as "not paid."

The country of origin pays the costs of sending the mail to the destination country. This includes transit fees through third countries. This is built on the assumption that countries tend to receive as much mail from each other as they send. With this balance of mail flow in mind, the GPU congress members focused on setting the foreign letter rates to cover the cost of sending the letter as well as the cost of the carrying and delivering the response within their own borders. This resulted in many countries setting the foreign letter rate at about or slightly more than double or 2.5 times the domestic rate.
The country of destination keeps all postage due collected. This is built on the assumption that countries tend to receive as much postage due mail as they send. Also, the objective is to eliminate bookkeeping as much as possible. In addition, another reason was to pressure postal administrations into encouraging prepayment of postage (a vestige of Sir Hill's original goal): they would bear the cost of sending the mail but would get none of the benefit.

The country of origin determines the amount of postage due - if any. The country of destination must comply with: a) the acceptance of franking; or b) the postage due markings of the country of origin.

The catch-all exception was stated in Regulations III of the Berne Treaty:
" Every postal packet which does not bear the stamp "T" shall be considered as paid to destination and treated accordingly, unless there be an obvious error." [Author's Emphasis].

Non-acceptance of the stamps issued by other UPU members for political purposes is against the rules, but has been accepted in the breach because of a catch-all phrase that postal administrations were not required to transport and distribute anything forbidden by local law or offensive to local sensitivities.

No extra charges, duties or taxes are permitted - on outgoing or incoming mail - except those explicitly enumerated and permitted by the Treaty.

This rule was an attempt to keep a level playing field for all members. The initial fears were that some countries could charge more as a barrier to trade if given the chance to make such determinations: a tool in a subtle trade war.

This rule forbad the use of postal tax stamps on foreign bound mail as well as postal tax postage dues on incoming foreign mail. While this, on its face, appears to preclude war tax stamps, countries got around this by simply raising their base rates within the margin the UPU range. ...

Each country decides what system of weights to use: grams or ounces. Whichever weight measure is used, each "rate" is considered equal. Roughly 15g equals ½ oz (a little over 14g.) for purposes of weight: both being the first rate level, followed by 1 oz and 30 grams were the second rate level. ...

The symbol for postage due is a "T" marking (the initial for the French word "taxe"), which is applied at the country of origin or the first UPU member country of entry into the Union. Every country was left to decide its own design of the "T" - which has become a vast field of study on its own. But the distinctive "T" markings have also allowed philatelist to identify a cover's origins when other markings were indistinct. Some "T" markings are extremely rare and there is still much to be done in researching and cataloguing this area. ...

Invalid stamps are to be noted by a "0" (zero) next to them, in the country of origin. In some postal administrations, a box is drawn around the invalid stamps; this procedure is accepted and understood. ...

At countries of destination, translations of postage due notations in centimes ("c.") are to be rounded up to the next unit of money in force in the country of destination. ...

Postmarks of the office of mailing are to be legible, rendered partially in the Roman Alphabet and Calendar, and should not be covered up by labels or the attachment of postage due stamps. The use of the Roman alphabet and western calendar appears to have been widely complied with among member states. ...

Mail is to be conveyed by the most rapid means available. The exception to this rule was mail of an "extraordinary" expense that had not been properly prepaid ...

Franking Mandatory For Certain Classifications of Mail. This was not necessary for letters, but mandatory on all other classes of mail in at least the early years. ...