If you will be exploring Florida outside of Tampa, adding Express Toll Unlimited to your rental is a good idea. That way you can breeze through toll areas without having to wait in line to pay.
It is generally allowed to rent a car one way between SIXT locations. Approval is required, so it is best to check with the station you wish to rent from before booking.
You can rent a car from the age of 21 at SIXT in the US. An underage driver fee applies to those under 25. Some specialty and larger car categories require drivers to be 25 or older.