First class and Priceless

First class noun - Mail that includes letters and postcards and packages sealed against inspection.

Priceless and first class are semantically related. in honorable topic. In some cases you can use "Priceless" instead an adjective phrase "First class", when it comes to topics like superior, excellent.

Priceless adjective - Having a value beyond any ability to appraise or adequately appreciate.

First class and priceless are semantically related. Sometimes you can replace phrase "First class" with "Priceless", this terms are similar.