Extra charge for multiple address shipping

[Rhobur]

Hi,

I need to add an extra charge for every two supplementary delivery addresses.

Ex.

For a single address delivery, no extra charge just the default shipping fee;

For a second delivery address I need to add a $xx charge.

If for ex the customer wants the products to be delivered at four addresses the extra charge will increase.

It seems I can't compute that in the cart class cause at that time I don't know how many delivery addresses the customer will choose.

Any pointers, please?

Robert