The best treatment for your double chin would be neck liposuction, and possibly a neck lift.
I have not seen any good non-surgical options to treat unwanted fat deposits in your neck. If your skin tone is good, and if you're under 55, neck liposuction may be sufficient. This is a minimally invasive, safe procedure with little downtime.
If you have excess skin in your neck, then liposuction followed by suspension of the skin via a short-scar facelift would provide natural, youthful rejuvenation.
I hope this is helpful for you.