The four-year deal with the subsidiary of Germany's Deutsche Post AG eclipses many of the main shirt sponsorship deals at rival clubs.
United's matchday kits are still sponsored by Chicago-based insurance broker Aon Corp. in a four-year deal worth around £80 million.
The latest sponsorship announcement on Monday came a week after it emerged that United's owning Glazer family have submitted an application for a partial flotation on the Singapore stock exchange to help cut the club's debts of more than £500 million.