Just in case this didn't come up in your search of the archives: the Helen Kellogg Library on Ave. Independencia (Gazcue) in Santo Domingo is excellent. PM me if you would like more details.
I used to order a batch of books from Amazon (US) about once an month, and receive them via my private mail box service here in the capital, P O Box International. The books would always arrive within a matter of days and the cost was acceptable: the mailing charge is domestic as Amazon only has to ship it within the US, and the P O Box fee was not exorbitant except in the case of very large and/or heavy books.
Once I joined the Helen Kellogg Library it was so good that I all but gave up ordering books online, but the option remains. If you have a network of English speaking friends it is easy enough to organise book exchanges and purchase new books for each other when someone is travelling or receiving a visitor.
Hope this has been of help.
Chiri