Since the effort to send an email, even a "personalized one" is rapidly approaching zero, sending a handwritten note is a very personal touch. If you want, you

Since the effort to send an email, even a "personalized one" is rapidly approaching zero, sending a handwritten note is a very personal touch. If you want, you can send postcards if you place them inside an envelope. I would not do it for new customers, unless the note was a brief typed note with a signature. Try your best to find unusual stamps - nothing says boring like a postal mark meter. Above all - make your notes sincere. That means for new customers, something short like "Thank you" because they just joined and for longer customers, a more personal note - "Thank you for being our customer for 2 years and one of our most helpful beta feature testers. Your feedback on X... (this assumes this is the customer's actual relationship). Good luck!