At the time of mailing, Porte de Mar stamps were placed (by the postal clerks) on the backs of envelopes in an amount equal to the required overseas postage and the mail was forwarded to Veracruz or Tampico. Mail packets were then placed on foreign ships to be transported to their ultimate destinations. The total value of the Porte de Mar stamps in each packet was calculated and that amount was paid to the ship’s captain (or agent) for carrying the mail. Arrangements were made with both English and French ships, and the postal patron had to specify which service was to be used.