Do I need to buy seed packets from Crazy Dave every time I want to use them?

The twin sunflowers and gatling peas are plant cards you can buy from Crazy Dave. When you buy these cards they are available at the start of the stage just like any plant card you've previously gotten by completing a stage.

In my playthrough, I bought the twin sunflowers when it was available. It takes 150 sunlight to upgrade an existing sunflower to a twin sunflower. You'd use this when you have abundant sunlight and scarce land. It takes 6 sun cycles to pay for itself, compared to 2 sun cycles for just putting another sunflower. In order to use it, it must be selected at the start of the stage and this costs 1 plant choice. Given the sun cycle cost and the plant choice cost, just planting sunflowers would have worked better so far. Saving for another plant selection slot would have been a better choice (I'm on the second night).

Once you buy or earn any plant in Plants vs Zombies, it stays in your inventory forever. Same thing with pool cleaners, extra slots and bought minigames.

Most things in crazy dave's shop are buy once use forever. All plants, cleaners, and minigames are buy once use forever. So, don't be scared to buy something from the shop. Fertilelizer, bug spray, and garden rakes are the only stuff that need to be bought again. P.S. please don't just read this, also read others if I miss out anything. P.P.S You will need cob cannon, gloom shrooms, twin sunflowers, and winter melons to survive far in survival endless. Trust me.