They have all info (prices, list of destination countries, etc) on their website. They also answer any questions on the phone. One of the ladies working there gave me her whatsapp number when I asked a question that she could not answer without talking to her supervisor who was not there at the moment, so I wrote to her later and got the answer. Very nice.
There's a lot of people commenting here but it's obvious that none of them ever tried to use this service. So they just repeat that there's no reliable postal service to send things out of DR. But actually this EMS service is a reliable service, and if I need to ship something else again, I'll ship it with them instead of DHL/FedEx/UPS for sure. They charge by weight only no matter what is the package size. I have seen packages that big in their office that I have never seen in any other postal office anywhere. I asked them can I send the box if its size is like a big fridge? No problem as long it's not more than 30 kilograms. So if you have to send something that requires a lot of packaging material (foam, bubble wrap, whatever) and it makes the package extremely big, no problem, they accept it.
Another good point with INPOSDOM/EMS is that they check the package contents in front of you. You show/explain what's inside to DNCD/customs agent there (who are actually very nice and friendly people, as well as all other people working there), they put a stamp that the package is inspected, and then you take your time to finally close it, seal it with tape, put the label on, etc, and nobody else opens it after that, because customs inspection is already done in front of you.
I'm not sure how it works from other INPOSDOM locations across the country, but it's not my problem really. Maybe in some in-the-middle-of-nowhere shItholes where many expats live, it will not work that good, but who cares. In Santo Domingo it works perfect.