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

It is buy once use forever, once you buy them they will always be available to you in your arsenal of extremely deadly plants.

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).