Add numericality validation for FlatPercentPerItem

app/models/calculator/flat_percent_per_item.rb

@@ -14,6 +14,9 @@ class Calculator::FlatPercentPerItem < Spree::Calculator
 
   localize_number :preferred_flat_percent
 
+  validates :preferred_flat_percent,
+            numericality: { message: :calculator_preferred_value_error }
+
   def self.description
     I18n.t(:flat_percent_per_item)
   end

spec/models/calculator/flat_percent_per_item_spec.rb

@@ -5,6 +5,11 @@ require 'spec_helper'
 describe Calculator::FlatPercentPerItem do
   let(:calculator) { Calculator::FlatPercentPerItem.new preferred_flat_percent: 20 }
 
+  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 "calculates for a simple line item" do
     line_item = Spree::LineItem.new price: 50, quantity: 2
     expect(calculator.compute(line_item)).to eq 20