You may also be interested in these guides: Can USPS Signature Confirmation Be Used to Ship to P.O. Box? and Guide Roundup – How to Ship on eBay and Paypal
No, USPS does not require a signature for any package or letter unless signature confirmation or another extra service such as certified mail is also purchased. Signature Confirmation costs an additional $1.95 when the shipping label is printed online and $2.35 when purchased at the Post Office. To read my extensive guide on when Signature Confirmation is required on eBay and Paypal see, How to Use USPS Signature Confirmation Effectively with eBay and Paypal.