FedEx delivery manager will send out e-mails to the customer typically giving a time frame on when their package will be dropped off.
For example, one of my recent deliveries(ground), it said "Between 9:10AM and 2:30PM"
They want it to be "Your FedEx courier will be there between 10:30AM and 11:00AM". I dunno how tight they want to go, but I could see a 30 minute window esp with ISR/DSR/ASR shipments.