text: Advanced Shipping Price Rules - Shipping Cost on Category Weight
  This rule is used to apply gradually increasing, decreasing or stable flat rate shipping rate to a particular range of category weight.
  In this, you have to mention the category, minimum category weight, maximum category weight and the amount which will be added or subtracted per kg. To keep the flat rate shipping cost stable throughout the range you’ve created, keep the fee amount zero.
  Screenshot - 1: (Admin: Cost on category weight based advanced rule)
  Screenshot - 2: (Shipping method front view)