Is there a Beijing post office to apply for a China visa?(I’m from USA and going to use a 144 visa

I’m applying for a 144 hour visa free entry and if I were to miss my flight leaving the city, if i need or want to stay extra days (for dental work), is there a post office I could request to stay longer by getting a regular ten year China visa when already in BJ? I thought I’d seen somewhere on this site that there was a central post office besides just the offices at the airport ? If yes, could you provide the central post office address? Thank you, Sincerely, Nida