The source is called open your mailbox and look at what’s in there. There is a massive industry designing, printing, shipping around pieces of paper that just get thrown away for no purpose. Some of them are outright scams (“Important Information! Open Immediately!” and it’s just an ad).
Shipping and packages account for 41% and First-Class mail is about 32%.
https://about.usps.com/newsroom/national-releases/2024/1114-...
Feel free to reply with your sources.