Paying $63 to collect probably $5 dollars in tolls is insane. The Turnpike Enterprise / Sunpass needs to find some better solution given the sheer number of tourists renting cars here.
You may want to investigate some other rental car companies, the major ones (Hertz/National etc) all charge $2.00 per day + cost of tolls, and the $2.00 charge is only for the days which you use a toll road. So if going down and back to the keys you would only need to pay for the first and last day here. Its all tied into the toll vendor the car companies use:
https://www.sunpass.com/rentalcar
and unless you are arriving late at night, you definitely will want to take the toll roads to get the keys, as driving the length of US-1 in Miami is a miserable experience.