Flat rate packages from USPS function similar to postage stamps. If your package is under a specified weight, it will get to its destination for one flat price. These prices are subject to change, and likely increase, at infrequent times. To not invalidate all the postage stamps someone owns whenever rate prices increase, the USPS has a "Forever" stamp that will hold its ability to ship your letter even if the rates on a stamp increase.
Similarly, Flat Rate boxes have the same functionality through their "Forever Prepaid" boxes. These are the same Flat Rate boxes USPS offers, but come with the shipping label pre-purchased and attached to the box when you purchase it. As long as you own the box, you can ship it without any worries if the package will meet postage requirements.
USPS Pages for ordering flat rate forever prepaid: