80 Centimes X 2 French Letter To New Orleans

Here is another one with an unusual 18 cents marking.

The 18 cents would be an accountancy mark I suppose??

The 18c is a French credit marking.(triple rate ?)
The letter was sent during the France-Usa postal Treaty of 1857-1869.

The most unusual feature of this cover is the red "Baltimore AM. PKT PAID" marking which is quite scarce and normally only found on inbound mail from the Caribbean. Sometimes on mail to US domestic destinations and some times on mail from the Caribbean to Europe. This marking means the cover first arrived in the United States at the port of Baltimore. Baltimore was an exchange port for international mails for only a few years.

Can we expand on that please? Not sure I understand correctly.

I'll have a go Rod.

It was a way of doing the accountancy under the treaties. New South Wales had something similar in the 1850s.

It was a way for the Postal Department to keep track of what was owed to whom.

The postal treaty of Washington (1857-1869) was complicated(I have it under my eyes).
In the cover above; the 2 80c stamps were at the double weight (2x7.5grams).Under the treaty;The French postal administration had to return 18cents to the American administration.Why? It all depended on the route and which countries ship (American,French) was carrying the mail.Also if it was via the British isles.Part of the credit could go to GB also.
The Americans would do the same thing on their end with letters going to France.
Calculations of the weight of each letter,departure,arrival ports;transit ports and the means of delivery complicated things.
Stampless letters had their specific credits.Insufficient postage also.
There was also a certain disatisfaction between the Postal regimes about who was working overtime more than the other.So they came up with this credit system to compensate.The Postal administrators of France and America would meet every 3 months or so and pay each others credit dues off... It chaged after 1870 into another treaty...

Thanks perf12 for the extra information

Thank you for the discussion so far. It has been very helpful.
dan

Based on the info here, I searched the book of Charles Hargest and found the info in the another chapter not related to French mail.