def customer_zipcode=(v)
      @customer_zipcode = nil
      if (v = sanitize(v))
        v = v[0..4]
        if v =~ /^\d{5}$/
          @customer_zipcode = v
        end
      else
        raise USPSValidationError, "#{__method__} must be a 5 digit number, found value '#{v}'."
      end
    end
def customer_zipcode=(v)
      @customer_zipcode = nil
      if (v = sanitize(v))
        v = v[0..4]
        if v =~ /^\d{5}$/
          @customer_zipcode = v
        end
      else
        raise USPSValidationError, "#{__method__} must be a 5 digit number, found value '#{v}'."
      end
    end
Zipcode of customer returning the package.
 According to the USPS documentation, Zipcode is optional
 unless <tt>address_override_notification</tt> is true
 and <tt>address_validation</tt> is set to false.
 It's probably just easier to require Zipcodes.
 <b>Required</b>.