49 C.F.R. § 209.109
Payment of penalty; compromise
(a) Payment of a civil penalty must be made via the internet at https://www.pay.gov/paygov/. Instructions for online payment are found on the website.
(b) At any time before an order assessing a penalty is referred to the Attorney General for collection, the respondent may offer to compromise for a specific amount by contacting the Office of the Chief Counsel.