What is the difference between these two shipping methods?

I wanna know what is the diff between;

-US Postal Service Express Mail Flat Rate Envelope

and

-US Postal Service Priority Mail Flat Rate Envelope



Which one is the one that comes normal in the mail. Thank YouWhat is the difference between these two shipping methods?
Both come in your mail. Express Mail arrives a single day after shipping and priority mail arrives 2-3 days after shipping.What is the difference between these two shipping methods?
Both are not standard mail.

Express Mail usually arrives between 3-4 days.

Priority Mail would take up to 7 days to deliver.