The stamps are to be used based on the guidelines by USPS. You can affix two stamps when the size of the envelope is big or you are sending a large letter. You have to pay extra postage costs when you send above the permissible labels of a particular envelope category. You can choose to follow the details as mentioned on the site of the USPS or Unites States Postal Services. They define the weight as well as the dimensions of the envelopes or the letters to ascertain the stamp that you will need to affix to mail the same.