The postage cost to send a letter to Mexico depends on the weight of the envelope, according to the U.S. Postal Service. Postage for a letter that weighs no more than 3.5 ounces costs $1.15 to $2.86. A letter requires between three and six 49-cent stamps.
Using First-Class International Mail, a letter that weighs 1 ounce or less costs $1.15, a 2-ounce letter costs $1.72, a 3-ounce letter costs $2.29, and a 3.5-ounce letter costs $2.86. This assumes no special mailing service is required. A postcard to Mexico costs $1.15 in postage. One can determine the exact number of stamps required by dividing the shipping cost by the value of a stamp; a First Class Mail stamp costs 49 cents. It is also possible to purchase the exact amount of postage at the post office to avoid overspending on postage.