29 CFR 801.43: Civil money penalties—payment and collection.

Where the assessment is directed in a final order of the Department, the amount of the penalty is immediately due and payable to the United States Department of Labor.

The person assessed such penalty shall remit promptly the amount thereof, as finally determined, to the Secretary. Payment shall be made by certified check or money order made payable and delivered or mailed according to the instructions provided by the Department; through the electronic pay portal located at www.pay.gov or any successor system; or by any additional payment method deemed acceptable by the Department.