Besides, it would be easy enough for an employee to cheat, say by marking his/her ballot the employer's way, showing it to collect the "reward," then either applying for a replacement ballot or simply correcting the original with white-out.

I suppose an employer could go so far as to require the employee to show a video showing the ballot, then showing the employee putting the ballot into the envelope, sealing it, and mailing it. But this would make compliance so difficult such an employer would be ratted out in no time.