FYI to everyone - If you go to that part of the country make sure to carry either your passport or birth certificate with you. When we visited we went by these checkpoints and they don't stop you on the way there...but on the way back you will be stopped at every one and asked to produce papers. If you're Dominican then your cedula should do. If you're foreign and you don't have your papers then be prepared to pay up money at every checkpoint. We didn't carry passports for my wife and kids and I had to do a lot of convincing and actually pay up at a couple of these checkpoints...They are just looking for some money, but they will hassle you if you're not prepared...
The best point you made is the very last. In the checks, if you had your papers it should have been enough, no problem. You got taken advantage of when you paid money, (you paid when you entered the country the first time, possibly paid again to leave if you left to Haiti, and then again to enter...you paid in immigration, why are you paying checkpoints!) but your comment shows that you realize that. I guess sometime it would just be better for you to pay a few bucks and keep it moving, especially if you're travel alone. Maybe good advice is to travel with a Dominican friend. Sometimes I don't make a lot of posts in DR1 because it seems there is a lot of insulting and offending the people and the country, but I will say something right now, a lot (maybe most) of the men at those checkpoints are bruto (example: one early morning we left in the madrugada driving from Jimani to the capital, my mom was laying down sleeping in the back seat of the car with a blanket over her head. The guardia asks if the person in the back is alive or dead. We were angry, because that is disrespectful, and I told him so. What if she was dead? It was inappropriate. My mom popped up angry and asked him if he is a doctor or a saint that can raise the dead.) As far Dominicans having their cedula being enough, of course you should always have your cedula, and everyone has them, but in those checkpoints, it's fine, open the car, open the bag if they really want to, but asking for cedula? thisIs basically insulting, if they are asking for your cedula, what's going on, is there a problem. The one time in my memory my husband and I were ever asked to show our cedula in a check (and this was recently) we said, "You show me yours, I show you mine." He told us cono and walked away. In the United States if a police stops you and asks for your ID while you drive along the street, isn't it because you've done something wrong? Dominicans really shouldn't be being hassled that far in the checks...irritating.