You are wrong, Larry. The rates are posted on a very large sign at the airport clearly visible to anyone taking a taxi. Additionally, every person who requests taxi service is told the rate by the driver before anybody gets in the taxi.
John, if by autobus you mean the guaguas (a few buses but mostly vans), then you have been paying more than you should. Up until recently it was only RD$10 from Sosua to Puerto Plata, but now it is RD$15. In any case, the RD$20 you were paying was the price for clueless gringos
If you want to walk to the highway with your luggage, it is true you can save money by taking either a guagua or a publico (public car) that goes between Sosua and Puerto Plata. To catch one, cross the highway and wait. When they see you standing there they will toot to see if you want a ride.