Transpacific airmail service from Manila to San Francisco was inaugurated by Pan American Airways on December 2, 1935. The rate for letters from Manila to San Francisco was 1 peso 50 centavos. On April 21, 1937 service was expanded with the end point being Hong Kong and rates were reduced with the rate for Hong Kong to San Francisco being 2.80 HK dollars and from Manila being 1 peso. Mail from other countries with transpacific airmail service requested was forwarded to the nearest serviced city and then flew on this route which was known as Foreign Contract Air Mail Route #14 or more simply, FAM14. All of this standard transpacific air mail during those pre-war years were flown on Pan Am "Clipper" flying boats. I do not know if there was a surcharge by the Indo China post office to get the letter to Hong Kong or Manila before it received transpacific air mail service.