Postage / Shipping issue with Bakery mod

[sparkdigital]

Fisrt of all thanks for this great module! Maybe someone can help me with the following shipping / postage issue:

My client sells a a small number of products at various prices. Postage is 7.50 for any three products or 9.50 for any 6 products. Customers can buy less than three or six products but they will still be charged 7.50 or 9.50. It is unlikely that anyone will buy more than six products at one time so this can be ignored.

Any ideas on how to set this up?

Many thanks,

Konrad