Postal service trucks don't have AC either. The least UPS and USPS could do is provide cooling vests to drivers to help them keep their body temps down. Fans won't do crap when it is 150 degrees in your truck.

The other thing they could do have the drivers go out earlier, start them at 4 am, get them done by 2pm at the latest. That way they spend less time in the hottest parts of the day and decrease the amount of stress on their bodies.