I am truly sorry of your experience in Luperon. Mine was the complete opposite as Luperon was one of the most friendliest ports of call including the Commandante of the Port ( who did not speak English). Fluency in spanish helped but is not required.
My concern is that you stated that you arrived Sunday night and went shopping on Monday morning. Did you follow the protocol of registering and establishing intent with the authorities before disembarking?
If you did not $210 was a very small price to pay. It could have cost you a few days in jail. I am certain that your encounter is a rare isolated incident and not reflective of the D.R. I spent two weeks there and had no problem or request for a 15% duty on any items purchased. We were on a 52'' Gulfstar with all the perks and were not searched or boarded at any time. We made a great deal of purchases and provisioned the vessel for an extended passage. I must conclude that there was something in your behaviour which resulted in being singled out.
Granted, No reason or excuse can justify your experience but the Dominicans are very strict especially when "drugs" are suspected.
Also there are "special" drug agents that work with the Navy and I "know" they can be very confrontational and corrupt.
Luperon is a great destination wwith a large sailing community and hope that this sort of behaviour with the local Navy does not continue.