The answer is YES!
However, due to the design of Happy Plugs True Wireless earphones, you can only use the RIGHT earbuds if you wish to use just one earbud. This is because the RIGHT earbud is the "Master".
So the correct steps to use only one earbud are as below.
1. Remove only the RIGHT earbud from the charging case.
2. Your phone/tablet should connect automatically to the earbud. If not, you may need to perform a reset.
3. Once you finished using, place the RIGHT earbud back into the charging case.
IMPORTANT: The LEFT earbud will NOT WORK if used standalone.
Note: If you also wish to use the LEFT earbud when the RIGHT earbud is already in your ear, you can do so. Just remove the LEFT earbud from the charging case and put it into your ear accordingly. Then you can enjoy stereo sound. When finished, place both earbuds back into the charging case.