I sent a small package to a DR1er in Sosua last month using USPS Priority Mail, and it got there in about 10-12 days. It fit in a Flat Rate Envelope [provided by the post office] and cost about $13 to mail.
Earlier this year I sent a small tyvek envelope to an address in Santo Domingo, and it, too, arrived in just over a week.
Honestly, I was pleasantly surprised with the delivery both times.
AE