Can I use letter stamps as postcard stamps?

No, you cannot use letter stamps as postcard stamps without additional consideration, as the postage rates for letters and postcards are different.

• Letter stamps, such as the Forever Stamp, are valued at the current First-Class Mail single-piece 1-ounce letter rate. As of recent updates, this rate is higher than the postcard rate12.
• Postcard stamps have a specific value designated for postcards, which is generally lower than the letter rate. For example, as of the last update, the postcard rate was $0.40 or another specified rate depending on the destination2.

To mail a postcard, you should use a stamp specifically designated for postcards or ensure that the total postage value on your postcard meets the current postcard rate. Using a letter stamp would result in overpayment for a postcard sent domestically, and it would not be sufficient for international postcards which have even higher rates24.