Forgot to file one of my 1099-B in 2017

Hi there,

 Today, I received a (legit) letter from IRS claiming that I sold some stocks in 2017 tax year, and did not report them.

After some investigation, I noticed that I did not entered the information for one of my 1099-B(*). 

  Those stocks were RSU (through my employer and I had paid %40 tax on them). The 1099-B says I had ~10K long term gain.

 

  The letter from IRS says I had $54K income, from selling the stocks and I have to pay ~$25K (for unpaid tax) + $4K (for Penalty and Interests). The letter also states that I have to either pay the amount by July 24th or say "I do not agree with some or all of the changes".

 

  I believe I should go with the second option. Wondering if I should file an amendment to my 2017 taxes, right now, or just send the document to IRS and wait for them to send me a bill?

 

Thanks,

 

* I imported the data from Charles Schwab through TurboTax (I had two accounts on Charles Schwab) and but TurboTax just imported the information from one them :(