Is there a way to do what the subject says?

I need to configure a certain product type for my client.

The box dimensions are 50x10x10, the initial weight - FOR ONE UNIT is 10 pounds, and each additional unit is 2 more pounds.

so for 5 quantity, it would be 10+8=18 pounds at those dimensions.

How do I set this up so that it passes the correct weight to UPS?