I made 3"x 3" diddy cards as my "thank you" notes using this adorable single stamp called Gossamer Lace on page 109 in the Annual Catalogue.
You'll notice that the Apothecary Accent Framelit was altered to fit the card - I shortened the label. The die is one of four shapes in the set (page 191 - Annual Catalogue).
The greeting is from my personal collection. For the little heart, I used the teeny heart from the owl builder punch (page 185 - Annual Catalogue).
Stampin' Up! also sell 7.9 cm square envelops which are terrific value - £5.50/€6.95 for 40 (page 160 in the Annual Catalogue). They're super quality too, and you can stamp on them and make them look pretty.
Here's the inside. The butterflies are from Make a Cake (page 44 - Annual Catalogue), and the sentiment is from Occasions Alphabet (page 147 - Annual Catalogue).