We have a number of products whereby x quantity fits into a box and are charged at rate A, if the number is greater than this amount or less then the rate is rate Y, how can we do this?

e.g. product x is sold at £8 each if qty = 150 (a full box) for any other amount it is charged at £8.50 each. IF customer orders 150 then charge is 150 * £8. If they order 177, 150 are charged at £8 each and 27 at £8.50. If they order
20 they are charged 20 * £8.50. If they order 300 they are charged 300 * £8.

Hope all this makes sense?

Matt