To give some finality to this discussion, the legal consensus was that the extension was 60 days total in addition to my 30 days on the tourist card, allowing me 90 days in the country.
I am not certain this will always be the case, but immigration officials at the border agreed, though they were a bit surprised that the document existed. Might have been the first time the officer in question even saw a prorroga. At any rate, they took the prorroga, made photocopies of it, and let me cross the border into Haiti. I did not have to pay the tourist overstay fee.
Just to confirm........you paid nothing for this extension which is supposedly 60 days over and above the tourist card and did not pay and exit fee either?
If so, it appears you were fortunate, because the DGM website says you pay for the extension.