I understand the concept behind Line Items, but can't figure out what I need to do to configure it properly.

I have a product, we'll call it "Membership" that costs $150. I want to put line items, "Extra Car Decals" which you get when you order membership. Each membership comes with 2 decals, but you can add additional decals for $15/each. I want the "Extra Car Decals" to display quantity and price in the "Memership add to cart" form. It gets slightly more complicated when there are 3 types of car decals: for minivan, truck or car.

I thought I would need to do this with Custom Line Item types, but I don't understand how.

Really lost, and would really appreciate some help!

thanks