I'm not an expert on the new biohazard rules, but we get formalin fixed
tissues mailed and UPS'ed in everyday and don't have a problem.  You might
want to pour off most of the formalin since the tissues are already well
fixed (just tell your pathologist so he knows they were originally in
adequate formalin) and it's recommended to add enough newspaper or other
absorbant to soak up anything that might leak out. A huge shipment might be
another matter to ask your shipping service about.

James Britt, DVM, DACVP
jbritt@arlpc.org
Livestock and Poultry Commission Lab
#1 Natural Resources Dr.
Little Rock, AR 72205
501-907-2435
501-907-2410 (fax)