Is there an employee at 421 8th Ave location on Sunday, 12/9 that can trace a lost letter that was sent certified mail?

The best idea to me is to go there personally on Monday.

Sorry, don't know. Try on Monday 12/10

They should be able to based on the tracking number. Speak to a supervisor if they don't answer to your satisfaction.

Yes, to an extent. It is tracked and they will do the best they can to find out where it is now based on its last location.

Go there Monday