First-Class or Third Class?
What's the difference between first-class postal service and third-class?
Carriers are required to move first class mail everyday, whereas with standard (i.e., the new PR word for "third class"), carriers have up to five days to deliver. Also, all first class mail that is undeliverable (missing zip codes, etc.) is required to be returned to sender - undeliverable standard mail can be trashed.
Now you know.