Having to deal with carrying mail would take away from the time, already short as it is, the truck would normally be travelling, not to mention the logistics of figurig out what truck is going where. And some trucks (most of them i guess), don't evn go through "trainsit hubs", but go point to point. And security could be an issueto, despite having sealed mail containers.

IMO, it is better and faster for the mail to use their own trucks between mail distribution centers.

It is only after verifying credentials of a reputed transport company the postal authorities can venture to take such a risk. Subsidies tothe Postal Department cannot go on for ever and ever, fossil fuels aregetting scarce.