I have had the issue a couple of times, so I made sure to include "Printable" in my title and state several times (in various ways) through the description that it is digital and will not be mailed. I also use the terms "Printable PDF" and "Digital File" as my materials used.
I looked at your items (super cute!), and I've noticed that "Printable" is the first word in all of your descriptions, AND you couldn't be more clear in your description that the item is digital. Hmmm...
You invariably will find the customers that just don't read the description, so I fall back on customer service. I don't offer a refund since they still got the digital file they ordered, but I do offer either a discount on my printed services OR a free digital item (of lesser value) that compliments the item they purchased. In my experience, just offering to give them something "for their trouble" makes all the difference and turns an irritated customer into a repeat customer.
I hope this helps! Have a GREAT day!
lisa