Sometimes you’ll build up long lasting, trusting and mutually beneficial relationships with some of your best “customers” (a person you do business with inside a company) you’ll even do business with these individuals across different companies. The times I’ve sent my customers something (be it a swag pack, bottle of drink or anything) it’s been really appreciated. A handwritten letter would be amazing! I did send a hand written thank you letter to a customer once (but the person is a good friend too so it didn’t count as much, although I thanked him for being a customer and wished him luck etc) and he was super excited and happy about getting a physical post card in such a digital age. It definitely is a good thing to do, and helps keep things human.