Great diagram! 
That helps the understanding of why the post isnt great but then surely travelling any distance - even collected by car has a fair old risk to it.
I assume the post office handles the packages with some level of care ( sometimes) because there are eggs that hatch and survive the postal service.
Someone collecting by car is usually going to be gentle with them, they'd be placed in the car and unless they go for a bit of impromptu rally driving or hit some serious potholes then the eggs aren't going to get too severe a treatment, they certainly won't be getting turned over etc.
The post on the other hand is a whole new ballgame, have you ever seen them take your parcel in the PO and DROP it into the collection sack?
Then every time it gets moved, checked, scanned, transferred etc the eggs get turned/flipped/jolted etc
The chalazae that suspend the yolk in place are basically just like two rubber bands, they suspend the yolk in place.
It is possible to twist them so tightly by turning the egg repeatedly in the one direction that they break, just like over tightening the rubber band in one of these model planes.
Thats the reason when turning eggs during incubation, its better to mark them on either side and turn them back and forward the same way between the marks, it stops the chalazae tightening up and breaking.
Certainly it does work, I regularly see people reporting 100% fertility/hatch rates from posted eggs so it can be done but overall there must be a decrease in the hatch rates to varying degrees in posted eggs