To post a letter-sized envelope from the United States to Northern Ireland costs $1.15 as of April 2014. At the current price of 46 cents per stamp, a letter would require at least three stamps. However, exact postage can be obtained from postal centers to avoid wasting money.
When calculating the price of postage, the post office considers the value of the shipment, the way in which it is packaged, the overall weight and the speed at which delivery is required. The website of the United States Postal Service provides an in-depth calculator that consumers can use to determine exactly how much various types of shipments may cost.