FBA fees for Products under $25

Hello, I’m looking at this new Private label product with price point between $16-19 And have Question about that.

I calculate with the FBA fees calculator my FBA fees for the Product is 3.72

I know if there cart is more than $25, I’m the one who pay for shipping.

But If customers buy this Product and their cart is under $25 amazon charge them shipping and to me(seller) both or only them? Do i have correct information or not?

And Also For Packaging i gonna use Ziplock(clear) bags, do i need any warning label on that.?

Thank you so much.