The Hawaii-inspired Aloha Shirt stamps are so popular that the U.S. Postal Service has begun printing them in booklets.

The stamps, first unveiled in January at a ceremony in Honolulu, have taken off as one of 2012’s most popular. Originally they were available in sheets of 20 and coils of 100. They sold more than 50 million in April alone. Now, they’ll be available in 10-stamp booklets.

Two of the five classic shirts depicted in the stamp art showcase surfers and their boards; one shows fossil fish, shells, and sea stars; another shows a tropical flower known as the bird of paradise; and one shows Kilauea, a volcano on the southeastern edge of the Big Island of Hawaii.

“The Aloha Shirt stamps have been a popular addition to the U.S. stamp landscape since their release earlier this year,” said Stephen Kearney, U.S. Postal Service Stamp Services manager said in a statement. “Aloha shirts are fantastic icons, colorful garments that will brighten up postcards sent from anywhere in the U.S.”