Agree the signs are annoying, was just trying to clarify the laws around putting things into a mailbox since the person above you was incorrect as well. I actually think since you put a stamp on it, what you did would be legal, but I'm not a postman. Here's the Mailing Standards of the United States Postal Service Domestic Mail Manual itself:

https://pe.usps.com/text/dmm300/508.htm#ep1051804

Except under 3.2.11, the receptacles described in 3.1.1 may be used only for matter bearing postage. Other than as permitted by 3.2.10, or 3.2.11, 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.