woocommerce flat rate shipping

I have a customer that is selling olive oil in 2 different size bottles. They plan on using only box size.
This box will hold 4 200 ml bottles or 2 350 ml bottles.

My challenge is when a customer orders more than what will fit in one box then it needs to add additional cost.

For example

order is 4 200 ml shipping equals $15
order is 5 200 ml shipping equals $15 (x2) = 30

Hope it makes sense what i am asking.

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