Do not pay for a rental GPS! It is a rip-off. Increasingly, even cheaper new cars come with their own GPS units embedded in the console anyway (rental agencies often won't bother upgrading/updating the cartographic base anyway), and if that is the case you can use them regadless of paying for outrageous navigation device fees.
Even if that was not the case, figuring out an unfamiliar navigation interface, maybe not set up in English, in an unfamiliar car, is really not something to look forward to!
Use a smartphone GPS navigation app instead, one you already know how to operate at home.