After years of using Fedex envelopes, I have to ask…
The rear of the envelope tells me not to use it for liquids, blood or diagnostics. The liquid and blood I can understand, but what the hell are diagnostics and why cant I Fedex them?
Well, they might mean other lab type stuff like stool samples and things like that.
And you CAN Fedex all these things, as well as radioactive material, hazardous waste, etc- just not in the Fedex envelope- use a Fedex box 
well because I dont know specifically what diagnostics are, im pretty sure i dont have any to send.
as BoBettie said- medical things like stool, cell samples, bloodwork, urine- stuff that the Fedex envelope is not good at protecting from damage. The envelope is mainly for documents and the like.
There are other requirements as well:
[ul]
[li]watertight primary receptacle[/li][li]watertight secondary packaging[/li][li]absorbent material[/li][li]sturdy outer container[/li][li]clinical pak as an overwrap[/li][/ul]