I see,
well I guess the client will have to find a compromise where he sets an average weight which is large enough for him not to loose money, in the contrary eveb earn a little bit money because he knows more or less how much a chicken weight in average, he just need to set a range so that all chicken weight are englobed within that range for a fixed price.
Example:
If a frozen chicken weight 200g, the owner could set an average weight called “from 200g up to 400g” so that even if a chicken is 300g, the owner still cover his shipping cost.