The problem with the USPS service is that when the regular mailman is off the sub may delivery the mail to your house where it will sit.
I used to use the P.O service but they could never get it right long term. With the move to temporary employees by the P.O. the quality has gone down and where I am we never have the same delivery person long term.
Did not work for us but it may in a small town where you know everyone