If the doc's you are sending to the Canadian Embassy are orginials then I would be sending them with DHL or FedEx. I have done this in the past and have had no problems. Costs about $80 with DHL in Canada and about $1,400 RD if you send from DR.
If it is a passport you want to send to Haiti - then the embassy recommends that you send it with the CDN gov't mail system between Santo Domingo Embassy and Haiti. I have done this too, and it worked but had some glitches - officials in Haiti said they never received it and Santo Domingo said they sent it. After about 1.5 weeks, the passport was "found" and there were no more problems after that.
To send doc's to a residence, I have used Registered Mail with Canada Post and my doc's have always arrived safely within 2-3 weeks. Costs about $16.
Where are you sending from? are you in the DR or in Canada? PErsonally I always used Fed EX. There are cheaper ways but hte costs add up if you start to factor in the costof long distance calls trying to track down mail or packages, the frustration and the time you spend explaining 17 times that yes you did send it......