Ha ha I went through that before and someone came up with the stickers "Radioactive" (they will buy that on a van or truck but not on a motorcycle, I'd think) and "Biohazard" or "Medical Waste". How about "AIDS Blood Samples Courier Association" ?
Or (example
"
R.C.M. Roadkill Chefs of Maryland". Make sure to have a little piece of squirrel tail visible.