I wish there was a bigger flat rate box, too, but I think it would end up being just as expensive, or even more so for zones that are closer to the originating zone. I myself think the new flat rate box is still too small to safely ship eggs. I'll stick to the #7 for now. They're deep enough to safely pack up to 18 standard eggs with plenty of packing.
FWIW, I've had eggs shipped to me in the small Priority boxes, but they were packed with either foam, or barely packed at all. I was amazed I only had one broken, BUT many of them had blown air cells. I think a bigger box with better packing would have prevented them from being beat up so badly.