The domestic and international surface rates were of 10c for up to 20g with a register fee of 20c.Cards and postcards were 5c for domestic mail (not sure for international).
   This rate was applied regardless of country of destination. We are aware also that early 1926 the 10c rate (which was current since 1917) was reduced for a short period of time to 8c (cards to 4c -?-). We don't know either the date of the reduction and the date of the increase to 10c again, between january and march 1927 (?).

all details for no air rates : look !

26 july 1926 : airmail from Mexico to US destination

          They are no domestic airmail . Mexico to Dallas by rail (to San Antonio from february 1928) , then flown to US destinations ,occasionally to Europe.

15 april 1928 : first regular domestic airmail service
        precise provisions were made for mail intended to be carried by air abroad. Article 4 of the official decree establishing Mexican airmail service states:
        "when an airmail article is also intended to be carried by a foreign airmail service, it will be charged a fee corresponding to the amount required by the foreign country in addition to the postage and the domestic air fee. If the air routes used during transitinvolve additional countries, the fee will be the total of the amount required from each country to use their air routes."
 

          It seems that these far sighted regulations were ahead of their time, as we are not aware of any country that flew Mexican mail other than the US. In this instance, a combination of the "flown only in the U.S.A." 50¢ rate and the new 25¢ domestic air fee was applied to give a total rate of 75¢ (the l0¢ surface rate was included in the 50¢ rate).

     At that time, the Mexican l0¢ surface rate was the same for both internal and U.P.U. destinations. Since the additional 25¢ air fee was only for the domestic air route, airmail items addressed to any part of the world and franked at the 35¢ rate (l0¢ + 25¢) were flown only in Mexico.
 
     On October 1, 1928 the Mexican and the US airmail networks were virtually linked at the Laredo/Nuevo Laredo border. The l0¢ surface rate plus 25¢ air fee flew items in both countries, and this 35¢ rate was maintained for destinations beyond the US. This mail was flown to New York and forwarded by surface.

  When the domestic and US destination rates were reduced to 20¢ on November 1, 1929, the 35¢ international air rate was  maintained.By then airmail and surface rates were consolidated, and items were usually franked with a single 35¢ airmail stamp. In June 1929Mexican airmail was accepted to be flown in Canada by the Montreal-Toronto and Montreal-Albany routes, and from August 1929 airmail was regularly carried to Guatemala City by air.As stated above, in both cases the basic rate was 35¢.

**via Brownsville-Miami (daily service)

*via Merida-Havana (weekly service)