Add shipping charges based on number of items sold

Hi,
my client sells one bottle of hot sauce on his site at the moment. all i am looking to do is have Shipping for
1 to 3 bottles be $5
4 to 6 bottles be $7
8 to 9 bottles be $9
..
12 bottles be free

How do I do this?

Please help me I have been googling for days.

Thank you,
Regina

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