I think emailing corporate was the right idea. The customer service people at my BJ's are absoluetely worthless-- so if I have a problem/issue I ask immediately for a manager--not a supervisor but manager. Not a department manager, assistant manager or the manager on duty, but the actual on paper store manager.
CSR's don't care because it is not their job-- unless they are the ones reconciling the drawer/receipts. Supervisors don't care because well, they were supposed to make sure someone staffed that lane but they didn't.
Managers care because it the sales figures and receipts are tied to them. Assistants managers may have a "not my job" mentality as well.
By emailing corporate, it shows the lack of concern or training that these customer service reps will have. It will come back to them.
If you felt like it, you could try to reach the actual manager and tell them that you informed the corporate office about fradulant transactions in your store and how the stafff didn't want to investigate. (That will get the manager's attention).