Received letter 12c

I recently received letter 12C from the IRS and it doesn't specifically ask for form 8962 as seems to be the most common scenario. IRS seems to be asking for:

  1. Copy of page 1,2 of the tax return
  2. Amounts and dates of all estimated tax payments
  3. Supporting docs verifying income and withholding
  4. Copy of the letter

I suspect it might be related to an estimated advance payment I made (1040ES). I was wondering what is the best way to send the receipt for this payment? I just have a confirmation number from IRS regarding the payment.