Alyssa,
I actually used Business Mail. They charge a nominal annual "membership fee" (only DR$200-300/yr) and charge per item received by weight (since that's how they are charged when they freight the mail in from Miami every day). Not sure what the present charges are. I remember paying about RD$5 per business size envelope, about RD$11 per heavy magazine or catalogue. Packages also come with the customs charges, but then, you'd have to pay that no matter what delivery service you chose.
I have no idea what EPS costs, although I heard it is a bit lower (but not by much).
You might balk at the cost, but frankly, for sensitive documents (financial, bills, etc.), and time-sensitive documents, the security you get using one of these services is worth it. The Dominican postal service (INPOSDOM) has improved immeasurably since I first moved to the DR in 1995, but there still are many things I would not entrust to them -- gift packages, popular magazines in English, checks and credit card bills. There are simply too many stories in the Dominican press about Postal employees being found having diverted such items. It happens in the US too, but I feel that USPS is overall safer and the US culprits get caught more often. Hopefully there'll be a day in the near future when I can trust INPOSDOM as much.
Hope this helps.
Regards,
Keith