I worked for UPS during Christmas season a while back and I can tell you that UPS delivers packages without getting a signature also. The only items we needed signatures on where the highly insured items and deliveries to some "certain areas". I would say 80% of what I delivered that Christmas no one signed for.
Both FedEx and UPS take into account the value of the item (declared or insured), the number of items stolen in the past in your area, and how busy they are.
Also, nobody can put anything without stamps into a mailbox. You buy the mailbox but it usually says property of postmaster general somewhere on it or the box.
If you were to leave a note for your friend in his mailbox without a stamp, you've violated federal law.
See below URL for post office reg and quote.
This is according to postal regulations at
http://pe.usps.gov/text/dmm/d041.htm
1.3 Use for Mail
Except under 2.11, the receptacles described in 1.1 may be used only for matter bearing postage. Other than as permitted by 2.10 or 2.11, no part of a mail receptacle may be used to deliver any matter not bearing postage, including items or matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle. Any mailable matter not bearing postage and found as described above is subject to the same postage as would be paid if it were carried by mail.
(Here are the only exceptions mentioned above)
2.10Delivery of Unstamped Newspapers
Generally, curbside mailboxes are to be used for mail only. However, publishers of newspapers regularly mailed as Periodicals may, on Sundays and national holidays only, place copies of the Sunday or holiday issues in the rural route and highway contract route boxes of subscribers if those copies are removed from the boxes before the next scheduled day of mail delivery.
2.11Newspaper Receptacle
A receptacle for newspaper delivery by private carriers may be attached to the post of a curbside mailbox used by the USPS if the receptacle:
a. Does not touch the mailbox or use any part of the mailbox for support.
b. Does not interfere with the delivery of mail, obstruct the view of the mailbox flag, or present a hazard to carrier or vehicle.
c. Does not extend beyond the front of the mailbox when the box door is closed.
d. Does not display advertising, except the publication title.