Sending a postcard is similar to sending a letter in that you must add the appropriate amount of stamps, correctly address the card, write your message, and find a spot to mail it. Part 1. Sending Postcards. Stamps must be affixed to each side of the card according to how long you want the card to take to reach its destination. First-class postage for a postcard is 1 ounce; priority postage is 2 ounces. Cards sent through the postal system account for about one-third of all mail sent from the United States. Most cards are flat, but some have three-dimensional shapes printed on them. These sometimes include people's faces which make them more attractive to potential customers. Cards can be written on both sides or just one. If you wish to send a card to more than one recipient, separate addresses with a line through each one. Also include your name and phone number if you have one. You may want to write a short note with the card if you have time. Find a mailbox or drop box and check the requirements for your area. Some require that you add an additional address beyond what's on the card. Others may limit the number of cards you can send per month or year. Always follow the instructions that come with any collection site you use.