Most of the time,the dieoff will be pretty bad when rock is shipped,UNLESS,they pack it to stay good and wet.Most online places will just stick the rock in a styrofoam cooler with some damp news paper throwed over it.
Theres a couple of things you can do.
#1 Invest in some dry base rock.A little over half my rock was dry base,that I've added over time.Just go roughly 1/2 base and 1/2 live.
#2 Pick up a couple of good sized rubbermaid totes,a couple of cheap powerheads,and a couple of small heaters.That way you can get rock online and cure it before it goes into your tank.You basically cure the rock the same way you cycle the tank.Just put it in the tubs with saltwater,add the powerheads to move the water,and the heater to keep it warm.When theres no ammonia or nitrites,the rocks ready to be moved to your display.