49 CFR §374.405

All motor carriers of passengers and baggage subject to 49 U.S.C. subtitle IV, part B, which provide in their tariffs for the declaration of baggage value in excess of a free baggage allowance limitation, shall provide for the declaration of excess value on baggage at any time or place where provision is made for baggage checking, including (a) at a baggage checking counter until 15 minutes before scheduled boarding time, and (b) at the side of the bus or at a baggage checking counter in reasonable proximity to the boarding area during boarding at a terminal or any authorized service point.