Variable productweight

Hi,
I downloaded Woocommerce about one year ago and I am very happy with it. Now we want to add the feature, that the customer can decide the weight of the product (clay) by himself/herself. Not only some variations of weight, but total freedom from 100g to 1000kg. Unfortunately I was unable to find a function to give the customer the ability to do so.
Additionally we want to include price discounts for buying a certain amount of kg/lb. (100g – 999g = 10€/kg; 1000g – 2500g = 8.5€/kg…)
I would be very thankful if someone could share his/her knowledge if they had a similar issue.

Thanks a lot!