Five minutes on the phone with Zac Posen and I’m sold. Sold on the Zac Sack, his new signature bag for the Z Spoke line, sold on his Z-Spoke “Sex Dresses” (we’ll get to that in a moment), and sold on the designer himself. For all his artistry, the former wunderkind is also a salesman. To wit: not only does the Zac Sack strike the perfect balance between uptown lady and downtown grunge gal, Posen declares, it can also make a great in-flight pillow for those tight economy seats. Really.

He may be eccentric, but he’s right about the bag. It’s the perfect size—no Olsen-sized monster or annoyingly tiny clutch for the Z Spoke girl—comes in spot-on colors, and has thoughtful details, like the row of grooves on the underside of its handle to make grasping it on your train commute that much more comfortable. Knowing a man might need assistance in imagining the myriad requirements an ideal bag should fulfill, he hired a female accessories designer (Yaz Buket, an Egyptian princess with a jewelry line at Colette), and used his devoted celebrity following as a testing audience.

“Every actress that came in the studio had to have a go with the prototype,” he told me, which is how Claire Danes, Alexa Chung and Dita von Teese were all spotted carrying the bag even before its release this week at Saks Fifth Avenue. And he credits his experience with Target with giving him the know-how to create a supple leather bag (lined in silk faille, no less) for $525, explaining it was the mega-retailer that taught him to “put what you can do into the product [versus advertising or packaging], and to make the right, intelligent choices.”