first class
first class {n.}
1. The first rank; the highest class; the best group.
The pianist was quite good but he was not in the first class.
2. The most expensive or comfortable class of travel; the best or one of the best groups in which to travel, especially by ship, train, or airplane.
Most people can't afford the first class when they take a long journey by ship.
3. The way of sending all mail that includes letters and post cards, anything written by hand or typewriter, and anything sealed so that it cannot be inspected, and that is the most expensive class of mail but receives the best treatment.
The usual way to send a letter is by first class.
Categories:
Source: A Dictionary of American Idioms