In the cash register I can purchase an item using one of my pink 5 stars. How does this work?

Not quite, you cannot purchase it with a pink star box. The only way to get that decoration is to open pink star boxes to reveal a Marvelous (purple) card, with odds of 1 Marvelous card in 10 pink boxes that you open. Sometimes it will take less boxes, sometimes more, depending on your luck.

Tip: To have more opportunities next collection set, save your pink star boxes to open next month, as the prize is determined by the current set when you open the box, not the time you receive your box.