Etiquette expert Jan Goss agrees—so much so that she always carries a baggie of small thank-you cards in her purse. “Never pass up an opportunity to leave a thank-you note.

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Unsurprisingly, birthday cards make up more than half of those sales. But thank you cards rank at third place, making the lack of options I encountered all the more confusing.

Whether your guests have kindly bought you things from your gift list, or you're just grateful to friends and family for travelling to celebrate with you, a thank you card is the perfect way to ...

Thank you for admitting when you are wrong. That is so right …” There are personal “thank you” cards from my Mom, aunts and friends. For gifts. For my help. For my thoughts.

As to what to write your thank-you note on, you don’t need fancy stationery or a card, Mr. Leighton said, although it’s a nice touch. A sheet of notebook paper will suffice.