Experienced Postmate Drivers?
Phone "GPS" is not Real GPS. Phones are 'located' according to the individual cell site to which they are connected at any specific time. That site can be up to a few miles or kilometers from where the phone actually is. Solving this problem seems to require that you use a real GPS device, and the more sophisticated the unit is, the better the results are likely to be. In other words, don't expect a "cheap" model to get the job done. Whatever GPS device you buy, it should be able to receive signals from three or four satellites, allowing it to precisely pinpoint your location.