I pre-paid for a car in Madeira last month from Europcar's own site (bmi's 15% discount was more expensive) and just handed the voucher over when I collected the car. Mind you, the upgrade wasn't much, I booked the cheapest 4-door car, Peugeot 206 or similar, and got a Seat Ibiza which was the next car up. I entered my DC number at time of reservation and was not required to produce my card. Amazingly the 1500 miles posted to my account within days of returning the car.