If you want to be sure they arrive you need to use Fed-Ex or UPS. Do not use USPS. I recently shipped documents from NYSTate to the DR. It will cost you between $40-50 dollars but it arrives in about 3 days.
If you think this you'll be sending things regularly, you should get one of those forward carriers mailbox. It's usually free or very cheap to set up and sending a document will not cost you more than a couple of bucks....