For $55, I'd consider the mod. How does it affect the warranty of the car?

Like any new Prius owner, I'm becoming more and more aware of my MPG. I get great highway mileage on my work commute (55+), but crappy on my evening errand commute. In the 20 minutes I spend getting dinner, picking up the kids, and picking up the dog, I'll lose 2MPG on the display, which I have to make up, on my next trip to DC. If I could run the first 5-10 minutes on EV, it could make a significant difference in my MPG.