NotLurking is correct, there are no USPS offices in the DR. Whatever is sent via USPS once it arrives in SD its given to INPOSDOM and they deliver the mail to your home or you pick it up.
It also works in vice versa. Anything sent from the DR to the US via INPOSDOM will be in the responsibility of the USPS once the package/mail is in US soil. In fact, I've received many packages and documents that was sent to me from the DR via INPOSDOM and even though the package have INPOSDOM stamps and such, the courier that brings it to my home is the USPS.
In fact, yesterday I had to go to the regional USPS office here in CT to get a document that was sent to me via INPOSDOM and USPS was not able to deliver the thing to my doorstep because they only wrote the name of the street but forgot to put the house number (duh!). After signing a yellow slip and showing some ID, I got possession of the mail.
I suggest to Ladybird to visit her local INPOSDOM office. In all likelyhood, her package was not deliverable for some reason (they missed a number in the address or misspelled something, etc) and its sitting at the post office in Sosua or in the main INPOSDOM office in Santo Domingo.
-NALs