As long as your shipping to a FedEx Ship Center, than the high temp is not as important as the overnight and early morning temp.
If you are shipping to a residence and the package is delayed, then the high temp will be vital, even if you have a guaranteed 10:30am time. We just had a "guaranteed by 10:30am" package delayed to a 1pm delivery last week due to a mechanical problem on the plane. Fortunately, the package was shipped "held for pickup" so the animal was unaffected by the high temp. It probably wouldn't have been delivered to a residence until 4pm.
I won't ship to a residence, regardless of the guaranteed delivery time, if the high temp would require an ice pack. The temp in the back of the truck is also going to be higher than the outside temp.