add drop down list for unit preference

app/models/calculator/weight.rb

@@ -4,7 +4,7 @@ module Calculator
   class Weight < Spree::Calculator
     extend Spree::LocalizedNumber
     preference :per_unit, :decimal, default: 0.0
-    preference :unit, :string, default: "kg"
+    preference :unit_from_list, :string, default: "kg"
     localize_number :preferred_per_unit
 
     def self.description
@@ -16,6 +16,10 @@ module Calculator
       (total_weight(line_items) * preferred_per_unit).round(2)
     end
 
+    def preferred_unit_from_list_values
+      ["kg", "lb"]
+    end
+
     private
 
     def total_weight(line_items)