Tiered cumulative commission by unit

I appreciate anyone that can help me with this. I have been working on this for a while and cannot figure it out. This is what I need. My company is doing a tiered cumulative commission by unit.

Units sold 1-3 commission is $50 per
Units sold 4-6 commission is $75 per
Units sold 7+ commission is $100 per

In January a customer buys one unit and the commission is $50. In February the same customer buys 3 units. The first two units would be $100 ($50 each), but the commission for the third unit would be $75 ($75 each). The total commission for that month would be $175. In March the customer buys 3 more units. The commission on the first two units would be $150 ($75 each), and the third unit be $100. The total commission for March would be $250. Please make any changes you want. The desired results is just for illustration.

Thank you to anyone that can help!!