Question: how do you know how many stamps you need for a package?
Answer: Use a Forever stamp for most standard mail items. A single Forever stamp will be enough to send off an item in a commercial envelope measuring 11.5 inches (29 cm) by 5 inches (13 cm) and weighing 1 ounce (28 g) or less. Anything larger will require extra postage, which you can add up using a postage calculator.