The airport rental car business in the DR is a scam I'm sorry to say although it's better now than say, when I remember it 10 or even 5 years ago.
You go online to an international rental car agency (Hertz, Avis, Budget, Enterprise, Eurocar etc.) and reserve a vehicle like you do in your home country and expect the company to be reliable with a car there when you arrive at the price stated.
#1. Those companies ,or at least as you know them in your home countries, don't exist in the DR. All of them are local franchises. Yes, every Hertz, Avis, Enterprise, Eurocar etc. are NOT corporate-owned locations. They don't exist in the DR market. Forget being able to contact the corporate agency and complain, it will get you nowhere.
#2. You will reserve (or worse, pay for) a vehicle online either directly through the major rental agencies website or an aggregator such as Expedia, Travelocity etc. You then get to the airport and you will be told the car is no longer available and the only one they have is double the price. Or you reserve an SUV and when you arrive there are no SUV's, only compact cars. This is the way it works. Online inventory for specific body styles doesn't exist for the franchises even though you can select them online when reserving the car.
#3. Even though the only legally required insurance is liability, you will be told you need to purchase other ones if you want the car. Often the insurance will actually cost more than the car rental. They count on having you over a barrel because you're stuck there at the airport (with bags, possibly a family etc.) and no other way out. I've noticed thing are getting better in this area but there's still far to go.
#4. Worst is if you pay in advance. Then you've already paid online (either to the international corporate entity or aggregator) and once they spring the local "surprise package extras" on you, if you back out, you have to fight for your pre-payment back.
#5. Once you capitulate and spend double on a lesser car, take photos of every single scratch, nick, dent etc. because when you go to return it, they will claim you caused the pre-existing damage and insist you pay for it else they'll charge your hold-credit card. Of course, if you pay the guy at the counter/inspecting your car, he can make this problem 'go away'.
The fact is that the DR has SOOO MUCH to offer outside the regular tourist areas but the car rental process is still corrupt and is holding back what would be a goldmine in local exploration tourism to the inner/south DR. Hopefully one day this will get tackled but for now, expect the above and be prepared...