I did one of those parking lots in February of this year and it was just fine. When we pulled into the lot, I paid and parked. The shuttle bus was right behind us and took us to the port in less than 10 minutes. We were actually running a little late and they actually called to make sure we were ok.
Upon departure from the cruise end, the shuttle bus was there in about 5 minutes and we were on our way home in no time.
Go for the savings on this one! The company we used was "Park N Cruise" and I just looked back into my old emails and sure enough, we paid $4.99 per night too!