If a person has a salary of 8,000 baht and a reimbursment of transport of 2,000 baht and earns a commission of about 10,000 to 30,000 baht per month (depending on his sales performance), how much withholding tax should be deducted from his salary since each month his income is different?

Can a fixed amount of withholding tax be deducted? How should the withholding tax be calculated?

5% just deducted from the total income. If you are providing service than 3% However as you mention you are on basic salary which is below Baht 15000 they cant deduct income tax on that. But not sure about your situation. So 5% would be acceptable.