Mail carriers are sort-of-federal employees and are not supposed to take cash tips or gifts over $20. So if you want to tip them, get them a $20 gift card or some cookies/candy. Well, gift cards are kind of a gray area, but are usually allowed. A restaurant gift card would be more acceptable than a Visa gift card, etc. Alcohol is not allowed on duty, even unopened in their vehicle.

I'm not sure about UPS or FedEx drivers.

But I used to deliver newspapers and really appreciated those tips!