Howto
Howto Export to Mono
The mp3 options channel mode is misleading. Even though it says "stereo", it will (can) eventually export a mono file. If the "use custom mix" options is turned on in the menu:
File -> Preferences -> Import/export
After the change then you will note an advanced mixing dialog will appear before the file is saved, and you can verify the file will be saved with 1 channel (i.e. mono).
In the next 2.1.3 version of Audacity when released there will be a "Force export to mono" checkbox for when you have a stereo track, but a mono file will still be exported from a mono track even if that checkbox is not enabled.