In this issue we will focus on the withholding tax on paying sales commission. 

Sales commissions are paid to employees or companies that sell merchandise in stores or by calling on customers in order to motivate sales people to sell more. A common example are commissions paid on real estate sales. 

The rate of withholding tax on paying commissions is varied, depending on "To whom you pay to" and "Where you pay to", therefore, we will classify as follows:

I. Paying commission to an individual which is Thai resident.

Paying sales commission to an individual (or even if they are an employee) is considered Income and this has the same tax liability as a salary. 
1.1 Paying commission to a non-employee individual.
The first time we pay, we can start with 5% withholding tax but if this person continues to receive commissions during the same calendar year, then we cannot continue the 5% withholding tax rate, but have to calculate the same as payroll tax for an employee. 
1.2 Paying commission to an employee.
Commission is considered as a kind of other income, therefore, we have to include the commission we've paid to the employee in the payroll tax calculation. This means that he or she will pay tax at a progressive rate, the same way as salary tax.
II. Paying commission to a Thai company. 

Paying commission to a Thai company is simple as you only need to withhold tax at 3% from the commission you've paid. 

III. Paying commission to overseas for a non-Thai resident individual. 

First, you have to understand that a Thai resident means anyone that stays in Thailand 180 days in the same calendar year. 
The withholding tax in this case is 15% unless that person is a the citizen of the country that has signed a double taxation treaty withThailand. If that is the case, no withholding tax is required. 

IV. Paying commission to a company overseas. 

The withholding tax in this case is 15% unless if the registered head office of that company is in a country that has signed a double taxation treaty with Thailand. If that is the case, the commission paid will be considered as "Business profit" under the treaty, and you must check if the receiver has Permanent Establishment (PE) in Thailand. If the receiver has no PE in Thailand, you don't have to withhold the tax. 
In general, having PE means having an office in Thailand or sending staff to work in Thailand more than 180 days in that calendar year. However, it also depends on country-by-country double taxation treaties.