To rent cars from most car rental agencies in St John's you need major credit card, hold a valid drivers licence (the names must match),
and be over 21 years old. Some agencies allow you to "cash qualify" but the paperwork is extensive and is not viable for out of town visitors
(check when you make your reservation). TIP: If the person who's name is on the credit card is not the driver, then add the real driver as a second driver
(there may be a small charge for that).
The unlimited mileage just does NOT happen with car rental companies in Newfoundland. I suppose its because the province is so big. Renting a car at one ferry terminus means a 14 hour drive across the province. Ex-pats and comefromaways complain about this all the time.
Many agencies have "weekend specials" but reserve a few days before long weekends. July and August are the busiest months of the year--reserve your vehicle well in advance.
Better Business Bureau tips.
Find Local Car Rental Offices.