The amount of money to give to a mailman for Christmas depends on many factors, such as how well you know the mailman and your financial situation. If you have a good relationship with your mailman and can afford to give them a generous gift, you could consider giving them a $20 or $30 gift card to a local coffee shop or store. If you don't know your mailman very well or your finances are tight, you could give them a small gift like a box of chocolates or a holiday card.