No purchase necessary. As a CPSC recall, Shimano is still obligated to replace them. Upon receival of your crank, they will ship a box that contains a new bottom bracket, crank, front derailleur, and chain.

If you go to a trusted shop, they can call a warranty in and have the crank shipped without sending your current one back (cross shipped). I would say that they would need to really trust you, because failure to return the warrantied crank in a timely fashion gets them charged for the warranty kit.