No, there is no legal requirement that a commission schedule be written--either in terms of the rate (including any credits, chargebacks, deductions, etc.) or in terms of the gross amount of sales; similarly sales targets do not need to be in writing. Also, an employer may, looking forward, alter the schedule, rate, target, etc., but cannot do so retroactively, for work done or sales made. Therefore, in the absence of a written schedule or a communication to the contrary, each quarter's commissions should be calculated the way the prior quarter's were--though again, with notice, that can be changed for the future at any point. If you think you are being cheated, you could take legal action (e.g. a lawsuit) if necessary.