What, like goo and secretions and stuff? (One day, I'll post my hilario-comedic Adventure of Giving Rude Specimens.)
Sending diagnostic specimens?
Use Safe Box
from Royal Mail
I can see that sending man-juice and snot through the post might be something some people might have to do. But is there really such a demand for such service that you advertise it on everyone's letters? Or perhaps this a response to it being quite common, and there being too many icky leaks.
(Me on the decline of the postal monopoly.)
* the letter itself was a contract for something exciting and as yet unannounced. Woo!