Postage rates increase often. Although advanced notice is given, it doesn't always leave time to use all the stamps purchased at current rates. The post office does make it possible to use old first class stamps by allowing customers to make up the price difference with additional postage stamps. Your letters and bills can be mailed with an old postage stamp beside another stamp that covers the amount of the rate increase.
Calculate the difference between the purchase price your old stamps and the current postal rates. For example, the price of first class postage stamps increased to 44 cents in May of 2009. Anyone holding stamps bearing the previous rate of 42 cents had a 2 cent difference in postage for each stamp.
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Buy stamps for the rate difference. Ask a postal clerk for stamps for the rate of difference between your old stamps and current postage rates. Buy a stamp at the rate of difference for every old postage stamp you still have.
Place an old postage stamp on a letter next to a stamp purchased for the rate of difference. For example, a 42 cent stamp next to a 2 cent stamp would account for the 44 cent postage rate set in May of 2009.
Buy "forever" stamps from postage dealers and use them for first class mail no matter how much the rates go up. The United States Postal Service sells books of stamps at current postage rates with a promise to honor them for first class deliveries at anytime in the future, regardless of rate increases.