This is a Sand Dollar (Encope michelini) echinoid found at a Gulf of Mexico beach in Florida. It has been cleaned. Note, the irregularity on one side is predation damage (possibly crab?) that the echinoid experienced while alive and it healed. It is not breakage.
Refer to the photo. The black centimeter cube is shown for scale and is not included. You are purchasing the specimen shown. Your purchase will include an ID label.