How do I enter pick and pack costs?

As a base amount per order plus an amount per additional item, which is how third-party logistics providers usually quote and how the work actually behaves: the first item costs the most, and each extra one is quicker.

It is effective-dated like every other cost, so a new contract with your warehouse prices the orders that followed it and leaves the earlier ones alone. Orders that ship in two parcels are charged per parcel, because the picker did the work twice.