Writing a proper thank you note, on paper and mailed with a stamp, really warms my heart. I love sending them as much as I love being a recipient.
When clients proactively add this item to a wedding suite, or give them as gifts to others, I have lots of fun imagining all the people whose days will be brightened when they open the mail to a hand-written smile from someone who cares.
Here's a recent set, designed as a wedding gift given to a bicycle-loving couple in the Pacific Northwest (printed by the always-amazing Margaret of Nightingale Handmade):