Yeah it's a basic curve...
At really low temps they can't metabolise and so they die.
And low to high they metabolise, but their metabolism speed is proportional to temp. So it gets faster as it gets warmer, meaning a shorter lifespan.
And at really high it kills them as it messes with lots of important proteins.
However in low temps for less than 24hours... where low will be well above freezing, most of the time they should be fine. Just not something I will be risking... I'd rather ship when it was definitely warmer. 10C and above is best.