It is completely legal: there is no requirement that an employer affirmatively provide proof, evidence, confirmation, etc. of payments to commissioned employees--that is, not unless an employee has a written contract requiring this. If you suspect you are being underpaid, your only recourse would be to sue your employer; in the course of the lawsuit, there are legal processes ("discovery") you could use to get access to this data, but you can only use them if you sue. And if you sue and the employer was paying your properly, you will incur the cost of the lawsuit and undoubtedly lose your job for not good reason.