49 C.F.R. § 215.411 – Reply.

(a) Within sixty (60) days of the service of a demand letter issued under § 215.409, the respondent may—
1. (1) Pay as provided in § 209.413(a) and thereby close the case;
2. (2) Make an informal response as provided in § 215.415; or
3. (3) Request a hearing as provided in § 215.417.
(b) The Office of the Chief Counsel may extend the sixty (60) day period for good cause shown.
(c) Failure of the respondent to reply by taking one of the three actions described in paragraph (a) of this section, within the period provided, constitutes a waiver of the right to appear and contest the allegations, and authorizes the Office of the Chief Counsel, without further notice to the respondent, to find the facts to be as alleged in the demand letter and to assess an appropriate civil penalty.