A friend of mine told me that he often catches the night bus from Merida, sleeps on the trip down. Arrives early (about 5am) to Chetumal. When the taxis stopped charging him 60 pesos and started demanding 200 pesos for the ride from the bus station to the Belize border he found out that there is a Guatemalan bus that you can catch for $5US instead. So he catches the bus, goes to the border. Customs is often closed this time of day so you typically only have to clear immigration. Then you can catch a cab back to Corozal.