Base Shipping Price but additional costs for additional items

[arthur graphics]

Hi!

I have a question about shipping rates and am looking for help. I have a client who sells art work. She wants to charge different shipping rates for different size items (11x14, 16x20, 18x24). But if a customer buys more than one 11x14 the shipping would only be an additional amount smaller than the original shipping amount.

Ex: 1 - 11x14 = $30.00 + (shipping) $10.00
    2 - 11x14 = $60.00 + (shipping) $10.00 + $5.00
    3 - 11x14 = $90.00 + (shipping) $10.00 + $5.00 + $5.00

    1 - 16x24 = $40.00 + (shipping) $20.00
    2 - 11x14 = $80.00 + (shipping) $20.00 + $8.00
    3 - 11x14 = $120.00 + (shipping) $20.00 + $8.00 + $8.00

I have been searching for a week and cannot find anything that will help me solve this problem. Please help me before i tear my hair out

Thanks!!!!