Mailing from coast-to-coast sounds like it would take a long time, but there are regular flights cross-country, and the sorting facilities bring mail to the middle of the coast, and then ship it straight across, east or west, depending. This means that the mail has to go first to a central sorting facility.
Truck and bus drivers must generally take a minimum of eight hours off between shifts, and their shift hours are often capped. Airline pilots are required to have 10 hours off between shifts to allow for transportation and rest time.
Unfortunately, for most professions there is no mandatory minimum of hours between shifts an employer may schedule (exceptions would include transportation and public safety kind of jobs). Most employers can require you to come back in less than 8 hours if they deem it necessary for business.