Fix Rubocop warnings and tidy up

app/controllers/concerns/order_stock_check.rb

@@ -4,9 +4,9 @@ module OrderStockCheck
   extend ActiveSupport::Concern
 
   def ensure_sufficient_stock_lines
-    if @order.insufficient_stock_lines.present?
-      flash[:error] = Spree.t(:inventory_error_flash_for_insufficient_quantity)
-      redirect_to main_app.cart_path
-    end
+    return true if @order.insufficient_stock_lines.blank?
+
+    flash[:error] = Spree.t(:inventory_error_flash_for_insufficient_quantity)
+    redirect_to main_app.cart_path
   end
 end

app/controllers/spree/paypal_controller_decorator.rb

@@ -60,7 +60,8 @@ Spree::PaypalController.class_eval do
     @order = current_order || raise(ActiveRecord::RecordNotFound)
     # At this point the user has come back from the Paypal form, and we get one
     # last chance to interact with the payment process before the money moves...
-    ensure_sufficient_stock_lines
+    return unless ensure_sufficient_stock_lines
+
     @order.payments.create!({
       source: Spree::PaypalExpressCheckout.create({
         token: params[:token],