additional box charge

Hi all, I have a problem with adding additional box charges to a customers order. I sell stained glass at various sizes and I would like to charge a box charge to extra large pieces. My problem is that each box can hold up to 4 pieces of glass for that one charge and a second box charge for additional pieces.

Example:

Customer buys 10 pieces of glass, 7 will need to be boxed in a big box. Each box can hold 4 pieces of glass. So I will need to charge for 2 big boxes.

Is there a mod. that would calculate when a customer has ordered a product that need additional charges and keep track of how many pieces before charging for another charge. Thanks for all your help.

Is there a mod. that would calculate when a customer has ordered a product that need additional charges and keep track of how many pieces before charging for another charge.
Ihave not seen one for this specialized application
Why won't per item with limits on # of items assist