Most people can relate to buying a book of stamps only to have the price of stamps change and then not being able to easily use the stamps. While the price of postage for a standard letter usually goes up, there have been a couple of instances where postage prices actually went down. Old, unused postage stamps don't expire and can continue to be used, although additional postage might be necessary.
Determine Postage Need
A postage stamp covers the cost to mail one standard-sized envelope weighing 1 ounce or less. The 2018 rates for the standard-sized envelope are 50 cents with an additional 21 cents for each additional ounce. This baseline goes up for large envelopes, starting at 70 cents for a legal-sized envelope. Standard postcards require 35 cents in postage. These are domestic rates. International rates for standard-sized envelopes start at $1.15 and go up based on size and weight.
Four to five pieces of standard computer paper mailed in a standard envelope usually fall within the 1-ounce weight. A thicker paper stock obviously weighs more. If unsure about the weight of the letter being sent, use a mail scale that is calibrated to current postage standards. Every US Postal Service (USPS) branch maintains calibrated scales to determine the exact postage needed based on current postage rates.
Buy Additional Postage
If you don't have enough postage on a parcel, you'll need to buy and add more. For example, if you found stamps in your desk drawer from 2001, the value per standard stamp was 34 cents, 16 cents under current postage needs. If you are in a rush and know your letter doesn't exceed the basic 50-cent postage, you could use two stamps, exceeding the required postage, and drop the letter in the mail. You won't get a refund for the excess, but you will save time and get the letter out without concern that it will be returned for additional postage.
Another option is to buy the additional 16-cent postage online or at the local USPS branch. If you purchase stamps online via the USPS, they are sold in sets, rather than individual stamps. This isn't practical if only a few old stamps are discovered. Stamps.com has print-on-demand services for additional postage, which is another option. Additional postage stamps start at one cent making it easy to make up any difference between the old stamps and current rates.
The Forever Stamp
The Forever Stamp first became available for purchase in April 2007 as the USPS attempted to help offset the continual changes in postage stamp rates. As its name suggests, you can use a Forever Stamp regardless of changes in postage for a standard letter. This means a Forever Stamp purchased in 2001 when postage rates were 41 cents are still completely valid for the current 50 cent postage rate. Use Forever Stamps without needing to buy additional postage.
Post-dated stamps are already used and no longer valid for new mail postage.