One of the biggest differences between the two services is that First Class mail doesn't have a default insurance policy like Priority Mail. So, in cases where a package is lost or damaged during delivery via First Class, the sender isn't covered.
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So the USPS moves about 146 BILLION PIESES OF MAIL EVERY YEAR, and about 3% of that mail is lost. The loss of mail is usually because the package was missing a proper zip code or the return address was not ledge-able so the letter/package could not be returned to sender.