Seed packets can be earned by using the desired plant in-game, but cannot be earned if that plant dies in-game 3 times. Seed packets can also be earned by opening daily gifts, and Pinatas.
Master Skills replace the leveling system from PvZ2. Master Skills are unlocked after beating Zomboss for the first time. Master Skills are unique to each plant class and boost a certain aspect of those plants. An example would be the Nut classes' 1st Master Skill which is Explod-O-Nut which allows them to explode when fully eaten. Master Skills require Skill Points which are used to obtain each skill needed for a Master Skill. An example of a regular skill would be the Pea classes' Heavy Peas upgrade which makes their peas push zombies back every 4th pea they shoot.