How can I mail perishable items?

Pack in dry ice and a form cooler

Depends on what it is. Dry ice is allowed as long as it is used as refrigeration. From the USPS guidelines: "Perishable matter is anything that can deteriorate in the mail and thereby lose value, create a health hazard, or cause an obnoxious odor, nuisance, or disturbance, under ordinary mailing conditions. Mailable perishable matter may be sent at the mailer’s own risk when it is packaged as required and when it can be delivered within appropriate and reasonable time limits to prevent deterioration. Examples of perishable matter include mailable types of live animals, food items, and plants."