A better option would be to clamp a weatherproof, toughened phone case such as an Otterbox to your bike and put your phone in that to use as a GPS / sat-nav. Best of all would be to use a dedicated toughened sat-nav such as a Garmin Zumo certainly if you are frequently wanting to use it such as being a courier when you could write the cost off as a business expense (depending on your local tax regime).