If you arrive at that time and are not saddled with a ton of luggage:
Take a cab into the city to the Caribe Tours station on Maimon Street (near the Jardines). They have buses to the Sosua area several times a day. You can check their schedule on the Internet.
The take a cab to wherever you are staying. A whole lot cheaper and you get some real "Dominican" experiences...
If you have perhaps an hour or so before your bus leaves, make a deal with the people at Caribe Tours to leave your bag behind the counter and go up to the Monument for a half hour look-see, its just a RD$300 taxi ride back and forth! Less than $4 each way and it's only a few minutes away.
Try it..
HB