I wrap each egg in toilet paper, then put a strip of bubble wrap over the eggs, close the carton, then I put bubble wrap under and over the carton. Do this with however many cartons I need and fill the rest with paper.
On my last 4 orders I have had like 2 eggs total break. One of the shipments was in a small box. If I had 5 inches of room above the eggs I couldn't even use the medium box.
I charge $12 for shipping because it costs me between $7 and $10 to ship plus cartons, bubble wrap, and any other materials. That comes out around $12 so I stick with that. I use the priority mail boxes based off weight and distance. The flat rates are not a good size. The small boxes will fit...