Accidentally claimed myself as independent on 2015 return, got letter from the IRS just now in November

I've been through this once before on the state level, which was resolved rather quickly. While going through the State return, I accidentally did not indicate that I was a dependent of my parents in 2015, as I only moved out on my own in November of 2015. Apparently this mistake was also made on the Federal return, as yesterday I came home to a letter from the IRS saying that I have claimed myself, while also being claimed by someone else.

I find it a bit concerning that this information took until November 2016 to reach me. I am hoping that the 3-year timeline outline applies to the date that I received the notice, rather than an earlier date.

In 2015 I started working in July, and lived with my parents up until November. I turned 19 in September 2015.

Currently I am living on my own with a pretty decent income. How much should the claimant correction in the 1040X form result in me being charged? And is there a possibility that I will be charged extra fees for this not being dealt with until now?