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.
The key requirements are that you must not drive: Without a break for more than 4.5 hours. After driving for 4.5 hours, a break of at least 45 minutes is mandatory. You can distribute that break over the 4.5 hours by taking a 15 minute break followed by a 30 minute break. For more than nine hours per day or 56 hours per week. This may be extended to 10 hours no more than twice during a week.