Your upstairs is likely a furnace by July. An attic fan pulls that trapped hot air out and draws cool air in, which means your AC doesn't have to work nearly as hard. We see the biggest difference on those first summer nights when the outside temperature finally drops.
Booking an electrician in late winter or early spring is the move. They aren't slammed yet, and you might catch a seasonal discount on the unit itself. The job usually takes a pro less than a day to cut the vent and wire the motor.
This 0% interest period is a hard deadline, not a gift. If you still have a balance on the card after the 18 months, you will owe interest on that remaining amount at the standard rate.