I haven't tipped the mail carriers for a few reasons... first is that we have a number of them and I never know which one is going to be delivering at any given time. I mentioned here a few weeks ago that I had to personally track down a mis-delivered package, so some of the carriers constantly put mail and boxes at the wrong place.
But secondly, they're not allowed by law to accept cash... and gifts only $20 or under. Per USPS:
"All postal employees, including carriers, must comply with the Standards of Ethical Conduct for Employees of the Executive Branch. Under these federal regulations, carriers are permitted to accept a gift worth $20 or less from a customer per occasion, such as Christmas. However, cash and cash equivalents, such as checks or gift cards that can be exchanged for cash, must never be accepted in any amount. Furthermore, no employee may accept more than $50 worth of gifts from any one customer in any one calendar year period."