Here’s the instructions from the West Virginia Department of Agriculture on submitting manure samples for analysis. (Emphasis added.)
Manure will expand as it travels to the manure lab, so never fill the jar to the top and make sure that the lid is screwed on tightly. Complete the submission form, sample date, producer name and address, manure type and storage type. This information is important for the lab. Mail sample as soon as collected or freeze the sample until it is mailed. Do not leave a sample at room temperature; the nutrient composition will change due to microbial action. To avoid the samples sitting in the post office over a weekend, mail samples on a Monday or Tuesday.
Indeed, U. S. Postal Regulation do permit the mailing of such biological materials as long as they are not infectious.
See 346.12 at http://pe.usps.com/text/pub52/pub52c3_023.htm#ep925060. International regulations also permit such mail.
UPDATE—I see that the Cabin Boy™ is tweeting various demands for apologies and is threatening criminal charges unless they are received. Perhaps he should review Maryland Criminal Law § 3-701 et seq., especially § 3-706, before he goes much further.
Keep Stop digging, Cabin Boy™.