Mail carriers are not supposed to accept gifts unless they are $20 or under, the amount the USPS allows. Avoid giving cash and instead go for a gift card in the suggested amount. I like to get $10 or $15 gift cards to my local coffee joint. I give these to the UPS and FedEx drivers as well. If that's too rich for your budget, baked goods or boxes of chocolates are great gratuity stand-ins.