If everyone wants later delivery, maybe that is something to look into (to meet/satisfy customer demand). Instead of the alternative of packages not getting delivered in the daytime and trying again the next day.
Now add the elastic capacity of a large number of casual part-timer workers who finished school/starbucks/whatever at 4pm and are happy to make a little cash working 5-7pm.
Now add customer preferences and you have a mess that can't be satisfied because everyone wants their parcel 5-7pm.