AFB,
its you that tells post office how to ship its your choice, flat rate shipping will take to long not good for plant.
Tell them priority mail 2 to 3 days.
I see its already to go , its a good idea to put on the top of soil in cup some damp paper or something similar then tape over it to hold it together.
Secure well to keep from bouncing around.
One pound and under is about 5.00 to ship priority mail over and i believe up to several pouonds is about 10.00, these are approximate numbers.
The faster plant arrives the better is my thinking as i have shipped priority mail and have not had any problems in past except once when i did not secure plant real well .