See the main fields and routines involved in calculation:
Customer File
The field IR Withholding Agent must be 1 – Yes.
TIO File
Tax File
Rate of 5%.
Income Tax Withholding
To make this calculation, the value of goods must be higher than the Withholding minimum value of this Tax, determined in the Tax Configuration Table. If the item total, with freight and expenses, is lower than this value, tax is not calculated.