Seed packets are packets containing the seeds of a specific plant inside it. It shows the cost in sun to plant the plant on the bottom of the package. The player gets one seed packet every time they finish a level (not counting the shovel, Suburban Almanac, Crazy Dave's Car Key, Magic Taco, Zen Garden, every ninth level of every area and after they finish the game). It does not have recharge written or signed, but it does have sun on it. Every seed packet says "Bloom and Doom Seed Co." on the front because that is the company that supposedly makes them. They fit in seed slots during gameplay.
Since the 3.9 update of Plants vs. Zombies 2, premium plants' seed packets have a unique look, with a gold outline and galaxy look for the center, and since the 4.1 update, world obtained plants gained their own seed packets as well.
Plants vs. Zombies Wiki has a gallery for Seed packet.