A standard letter envelope that weighs up to one ounce requires 49 cents in postage as of August 2014. The United States Postal Service also allows customers to use one forever stamp to mail letters weighing up to an ounce.
The shape and size of the envelope, the weight and physical characteristics of the envelope contents, the travel time you give to the mail carrier to deliver the envelope and whether or not you arrange for insurance on the contents and other special handling requirements all factor into the postage rate and number of stamps required to mail a particular letter. There is a substantial increase in the cost of postage if the carrier transports the envelope by air mail. Specific rates and handling requirements can be determined at a local post office or on the USPS website.