checkout_controller_decorator.rb

Spree::CheckoutController.class_eval do
  def ensure_checkout_allowed
    check = @order.checkout_allowed?
    unless check == true
      redirect_to spree.cart_path, :flash => {:error => I18n.t("checkout_allowed_errors.#{check}")}
    end
  end
end

order_decorator.rb

Spree::Order.class_eval do
  def checkout_allowed?
    return :not_empty     unless line_items.count > 0
    return :minimum_value unless total >= 10
    true
  end
end