I don't know how efficient the postal mail system is, but if you mailed it 7 days ago, how confident are you that it's even arrived yet?

I just had an itinerary be ticketed for which I mailed a voucher from San Francisco to Detroit (so no country borders to cross). Date of mailing: 12/4. Date of actual ticketing: 12/19. Maybe I should complain about how United treats its US-based customers!