The conveyance of postal items originating from Switzerland by a Zeppelin airship was possible from the earliest days of zeppelin mail delivery. Up to the first attempt to fly to North America of the airship LZ 127/Graf Zeppelin (May 16/17, 1929), however, Swiss postal items had to be mailed to a country that was eligible to directly send an item by a Zeppelin airship. Of course, the easiest way was to send a postal item to Friedrichshafen, the home base of the Zeppelin airships just adjacent to the Swiss-German border, where it received the necessary (German) stamps for the Zeppelin flight. The German and Swiss postal authorities then ratified a contract that allowed the direct shipment of Swiss postal items without the need of an intermediary country. Thus, from the second attempt to fly to North America on August 1, 1929, Zeppelin mail only carried Swiss stamps and a mixed franking was not anymore necessary. The contract was announced on July 15, 1929, on the Swiss daily newspaper "Basler Nachrichten":
Basler Nachrichten, July 15, 1929, Morning edition
Zeppelin delivers Swiss mail
According to a an agreement with the German postal authorities, it will be possible to use the flights of the airship "Graf Zeppelin" to deliver non-registered letters and post cards, mailed in Switzerland, to the United States of America or to a country on the way to New York. The weight of a single letter shall not exceed 20 grams. The total postage for a post card is 2 francs 65 cents [rappen] and 5 francs for a letter. The postal items need to carry a noticeable annotation "With airship L.Z. 127 to America" and be completely franked with airmail or regular stamps. Not or not sufficiently franked items will be returned to the sender. The German postal authority does not guarantee that the items will not be transported by any other mean than the airship to America. No refunds to the sender will be made for items delivered to the airship. Any postal item to be carried on the America flight with the airship "Graf Zeppelin" will be mailed on a usual way to the postal office "Romanshorn 1", which will deliver the items to Friedrichshafen five times a day.
Basler Nachrichten, July 15, 1929, Evening edition
Before the second America flight of “Graf Zeppelin“. Airmail agreement
As "LZ 127" will restart her long-distance flights in early August, it might be already today be of interest, to hear about an agreement, which has been taken by the German and the Swiss Federal postal authorities. According to this agreement, it will be possible to use the flights of the airship "Graf Zeppelin" to America to deliver non-registered letters and postcards to the United States of America or to countries on the way to New York (Mexico, Costarica, Cuba, Guatemala, Panama, Salvador, Columbia, Ecuador, Peru, Venezuela, Canada). This airmail shall not exceed 20 grams of weight. The total postage is 2 francs 65 for a postcard and 5 francs for a letter. The items will be mailed on a usual way exclusively to the postal office "Romanshorn 1", which will deliver the items to Friedrichshafen. The postal items that were already mailed for the cancelled flight will receive an extraordinary stamp carry the words „Beförderung verzögert wegen Abbruchs der ersten Amerikafahrt. Neue Postsendungen werden unter den bisherigen Bedingungen aufgenommen“ (=Delivery delayed because of the interruption of the first America flight. New postal items will be accepted under the existing conditions).