I have shipped larger individuals 1500 grams to 1800 grams (3 and 4 of them in the same box) without any issues, I also have shipped larger and smaller individuals together without issues.
The key it's to really know what you are doing, how to ship, when to ship and when not to ship.
To the OP do things happen? Yes they do but they are rare and even if the carrier delays your package and the package has to be redelivered late, if the person shipping knows how to pack and does it well chances are it won't even be an issue (I know Fedex lost 2 of my packages in the past 6 months and both still made it just fine.