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While you can purchase postage stamps at discount warehouses stores such as Costco to save a few pennies, it's possible to get postage stamps at more than 10% off their face value by purchasing them through stamp dealers such as hgitner.com:

Discounted postage, simply put, is US postage sold under face value. This postage is perfectly legal to sell, buy and use on all your USPS mail. The secret is that the volume of stamps sold to collectors over the last 50 years is enormous: often much larger than the demand from today's collectors. When these stamps were purchased, the post office received full face value for them. Today, when large accumulations come on the market, our position as a one of the largest US mint stamp dealerships allow us to buy them in bulk at current market prices, often below face value...The stamps which have not appreciated in value to collectors, we can now offer to you for your postage needs at less than face value.

Stamps do have to be purchased in lots with a face value of $100 or more and it's not possible to choose the exact stamps you want. Still, these deals are excellent for anyone who sends a lot of mail, and especially for those who do so as part of their business. Simply purchasing stamps from a new source can instantly reap double-digit savings on shipping costs.

Source: Tip For Buying Postage Stamps At A Discount