I hold an IT position in a company that ships out products somewhat similar to the way VZW might. The tracking numbers are placed on the boxes most likely by an automated conveyor belt system. The system is automated to tie the tracking number to the order, but that doesn't mean the customer will be able to see it right away. Oftentimes there will be a database batch process (stored procedure or perhaps a data warehouse update for fellow geeks) that will update the sales system from the shipping system.
'Not saying this is definitely true, just making a point that bigger companies are not necessarily "real-time" about all of this, for various internal reasons.