As of Sunday, July 9, 2023, the cost of a First-Class Mail Forever stamp is $0.66. This is an increase from the previous cost of $0.63. The price of a Forever stamp will never expire, even if the First-Class postage rate goes up. The cost of sending a standard-sized, rectangular envelope with a First-Class Mail Forever stamp starts at $0.66, while the cost of sending a standard-sized, rectangular postcard with a First-Class Mail Forever stamp starts at $0.51. The cost of sending a large envelope (flats) starts at $1.35. The maximum weight for First-Class Mail letters is 3.5 oz, and for large First-Class Mail envelopes, the maximum weight is 13 oz.