The Tarifa de Portes published dated Jan 1, 1895 shows the Tarjeta Postal rate to be 0.03 for International Service. Same in 1897.
This rate is repeated in the publication La Ciudad de Mexico by Adolfo Prantl and Jose L Groso, published 1901, sort of a businessman's guidebook to Mexico.
The next published rate I have is from a decree published Aug 3, 1915, in which the rate is now 0.04.