Display price per Kilo without charging for product

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I am looking to display the price per Kilogram on products with various weights. However, when the customer adds the product to their cart I don't want to charge them until they pick up the order. We sell a variety of sausages that people pick up at farmers markets. An example would be our pork sausage is $21.99/kg, Each package costs aproximately $8-$10. How can I display $21.99/kg and charge the customer $0.00?