I don't know if the heat is so much a problem as the rough handling. I got advice on heat treating eggs and looked it up online. And it appears that you heat the eggs for 114.8 for 11 to 14 hours. I doubt the eggs get that hot in shipment. A batch of eggs I shipped and it gets hot here in NC, the client reported back that 7 out of 8 were fertile and 6 out of 7 hatched. They were shipped to Texas also a hot climate.
I pack my eggs in the 12X12X8 priority mail box. Wrap the egg twice with 12X3 strips of bubble wrap, then twice with paper. They usually end up being double or triple the size. Eggs are then either suspended in packing peanuts or shredded paper. I have been getting great reports back on the excellent shape they arrive in. And on fertility.
The biggest letdowns I have had on eggs were when I received eggs in the tiny priority box, poorly wrapped. If you are want and she does not mind the extra weight go to a bigger box than the 12X12X8. Express mail is handled quickly and in a hurry so they may get rougher treatment. The eggs are good for couple weeks so a few days to collect and a few days to ship is not as important as the handling.