it would require quite a bit of work to do this using ecart, and this modification would not be supported.
you would need to add a new column to the products table to set the additional shipping amount.
you would need to edit the products insert and update page to add new form fields for the additional shipping amount.
then use data assist to update the Insert Record and update record behaiovors to store the new shipping amount in the products table.
next edit the eCart object to create a new column named "shippingAmount".
also create a shipingTotal calculation as:
[shippingAmount] * [Quantity]
Next update the eCart Add to Cart button behavior on the products detail page, bind the shippingAmount cart column to the new shipping amount column in the products recordset.
then edit the ecart object again to set a new shipping rule to use the subtotal of shipingTotal calculation.
honestly, it would be simpler to upgrade to PS4.