5-12.6 Penalties A penalty amount (calculated, as a matter of policy, in accordance with Office of Management and Budget [OMB] regulations) will be paid, in addition to the interest, if: The supplier is owed interest. The supplier is not paid the interest within 10 days after the date the invoice amount is paid. The supplier makes a written demand that the Postal Service pay such a penalty not later than 40 days after the date the invoice amount is paid.