code: private static String trackingNumberURL(String carrier, String trackingNumber)
  			throws AuthenticationException, InvalidRequestException, APIConnectionException, APIException {
  		try {
  			return String.format("%s/%s/%s", classURL(Track.class), urlEncode(carrier), urlEncode(trackingNumber));
  		} catch (UnsupportedEncodingException e) {
  			throw new InvalidRequestException("Unable to encode parameters to " + CHARSET
  					+ ". Please contact support@goshippo.com for assistance.", null, e);
  		}
  	}
docs: Return URL that maps given tracking number on given carrier as
  https://api.goshippo.com/tracks/<i>carrier</i>/<i>tracking number</i>