Add numericality validation for FlatPercentItemTotal

app/models/calculator/flat_percent_item_total.rb

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

spec/models/calculator/flat_percent_item_total_spec.rb

@@ -8,6 +8,11 @@ describe Calculator::FlatPercentItemTotal do
 
   before { allow(calculator).to receive_messages preferred_flat_percent: 10 }
 
+  it do
+    should validate_numericality_of(:preferred_flat_percent).
+      with_message("Invalid input. Please use only numbers. For example: 10, 5.5, -20")
+  end
+
   it "computes amount correctly for a single line item" do
     expect(calculator.compute(line_item)).to eq(1.0)
   end