GM Financial - I want to buy out my lease - first time doing this so multiple questions

Hi, I have a 2019 Buick Encore and am in the last month of a 6-month lease extension. My payoff is $16,100 and it looks like I can sell to CarMax for 21k. I’d like to do this but am a bit lost. Thank you in advance for any help you can provide! I really appreciate it. My questions are:

-It looks like all of this has to be done by mail, is that right? And only by check?
-The delay for title seems to be the biggest time issue. My Carmax offer expires in 10 days. Any way to speed the process up?
-Is there any way to do this all in person to save time?
-Anything I need to worry about on the Carmax side?
-Any hidden costs on either side that I’m not seeing or aware of?

Again, thank you so much! I apologize for the multiple noob questions. Just trying to figure out how to get this done as smoothly as possible.