I second the idea of getting the SunPass. I got one for a trip to Keys as well. I do not recall having to get it in person. The SunPass site indicates it is still available on line at a price of $4.99 and will be mailed to you 4 to 5 days after you apply for one. I am referring to the "Mini-Transponder" which goes on the windshield below the rear view mirror alongside, and in addition to, any other toll tags such as the TxTag, which I have here in Texas. You preload the account with $20 or so and give them a valid credit card, which allows them to replenish the balance as needed while you use it. After your trip, I recall you can request your account be closed and the balance refunded, usually something on the order of $10-$20. I have left mine open and used it again on the another trip to the Orlando area. It basically works in similar fashion to turnpike networks around the country, such as Texas, Kansas and the Northeast.
I found that having one is handy, not only on the Florida Turnpike, but on several of the toll roads around Miami and Alligator Alley (I-75 between the Miami metropolitan area and Naples).