Add numericality validation for PerItem

app/models/calculator/per_item.rb

@@ -10,6 +10,9 @@ module Calculator
 
     localize_number :preferred_amount
 
+    validates :preferred_amount,
+              numericality: { message: :calculator_preferred_value_error }
+
     def self.description
       I18n.t(:flat_rate_per_item)
     end

spec/models/calculator/per_item_spec.rb

@@ -7,6 +7,11 @@ describe Calculator::PerItem do
   let(:shipping_calculable) { double(:calculable) }
   let(:line_item) { build_stubbed(:line_item, quantity: 5) }
 
+  it do
+    should validate_numericality_of(:preferred_amount).
+      with_message("Invalid input. Please use only numbers. For example: 10, 5.5, -20")
+  end
+
   it "correctly calculates on a single line item object" do
     allow(calculator).to receive_messages(calculable: shipping_calculable)
     expect(calculator.compute(line_item).to_f).to eq(50) # 5 x 10