Hey gang, if you can afford $3 per order (even for certain promotional periods), there is a great solution to send your customers a handwritten (actually written by a robot using a real ink pen) thank you note. You will need to set up two accounts...one with Zapier (a great middleware solution that brings logs of apps together with Shopify) and Handwrytten.com.
In Zapier, you set up a "Zap" to trigger when you have a new order (you can choose the payment and fulfillment statuys). I chose to trigger when orders are shipped. You use the content from your Shopify order to populate the name/address of the thank you card. Write a standard "thanks for your order" message and include any personalizition such as first name from the order.
Now, each time an order is shipped, this integration will fire and a handwritten thank you note will be sent to your customer. You could use this for certain promotions or just as you're growing.