First class mail and standard shipping are different.
The first class mail is for mail such as postcards, letters, large envelopes, and small packages.
Standard mail refers to mail such as flyers, circulars, advertising, newsletters, bulletins, catalogs, or other printed pieces that are generally identical in content.
First class mail is always given priority for shipping over standard mail and first class mail usually arrives much sooner than standard mail.
Standard mail is sometimes slower than first class mail but not always.
Packages are always considered to be first class mail and usually get to your address within 3 to 4 days while standard mail can sometimes take as long as 7 days to get to the destination.
However in some cases standard may may get to you at the same time as first class mail.