Gift for my mail carrier

Hi all! I'm not a USPS employee so I hope it's ok to post in here but I wanted some advice on giving my mail carrier a little something for Christmas. I have a small online bookstore and my carrier has obviously been instrumental in getting my customers their orders in a timely fashion and I'm just so thankful for the work he does and that all of y'all do! Anyway, are there any rules or parameters that I need to be aware of in regards to gifts? What have been some of your favorite things that you've received from customers?