Caribe Tours does not cover the eastern region, unfortunately. You have to take the Expreso B?varo from PC to SD.
Their B?varo/PC stops are in Friusa - near Politur, and in Ver?n - opposite the Banco de Reservas.
Departures at 7, 9, 11 am & 1, 3, 4 pm. Takes about 3 hours. Fare is about RD$400.