def shipping_cost
      currency = 'USD'
      shipping = @contents.inject(0) { |total,item|
        total + item[:regular_shipping].to_i
      }.to_s
    end
def shipping_cost
      currency = 'USD'
      shipping = @contents.inject(0) { |total,item|
        total + item[:regular_shipping].to_i
      }.to_s
    end
Returns the shipping cost for the contents of the cart.