I ship to the US using INPOSDOM on occasion, I mostly use “correo certificado” or if it’s too heavy “encomienda postal”.
For correo cerrificado you just need a tiny form detailing the box contents and value. For encomienda you need a larger form with more info.
They both take about two weeks to arrive. Sometimes they are delivered in 5 days others in about 25 days.
If the item you’re sending is of high value or you want it there faster, use EMS (another inposdom service) they guarantee 5 day delivery and is generally more secure (but more expensive)
On a side note, none of those services are allowing the shipping of any kind of cellphones. I tried to ship one to canada some days ago and was told it’s not allowed. So if you’re shipping expensive electronics they might turn you down. If that’s the case I suggest you go to UPS. They’ll charge you 70usd and up, but the item will be there in 2-3 days.
Lastly, I don’t suggest shipping anything INTO the DR without a courier, as things seldomly arrive. Shipping out using INPOSDOM is fine. Shipping in not so much.