I like simple thinks thats why I made this more simplistic looking thank you design. 
You can add a line to write the buyers name, care taking instructions or social media chanels. Here are a lot of opportunities. 

I imported the font as it's a Adobe Font but you can write whatever you like inside XCS. The frame is from the XCS Library. I just aligned them both middle.

I used the tiny circles inside the file on the top to first try how it's performing on my shipping box cardstock. 

Adjust if you want more visible scoring but keep in mind too much can break you package or make it less secure for shipping.

As title says fold up you shipping box and pack the next order for your customer.
 

I think thats a cute addition to create a wow effect at customer end. This also could be a great alternative to classic paper thank you cards. 

What do you think about this? Write in the comments! ~Anzy