The price of a postage stamp is about to go up again. This Monday, the price of a stamp goes up a penny. It will now cost to 42 cents to mail a first class letter. Monday also marks significant changes in U.S. Postal prices for shipping services depending on your destination. Priority Mail will have an average 3.5% savings for customers who use electronic postage or meet other requirements. And parcel return service will move entirely to a weight based pricing system, resulting in significant price reductions for many lighter packages.