Tax compliance when paying abroad
As an example: If you are going to pay a UK company for software licensing, normally the withholding tax rate would be 15% because software licensing is considered a kind of royalty. However, under the Thai-UK DTA you need only deduct tax of 5%
If you are undertaking transactions that are not classified as either dividends, royalties, interest or capital gains, these are considered "Business Profit" under the DTA. You will need to check if the company you are paying has permanent establishment (P/E) status in Thailand, if the company does not have P/E in Thailand, then you don't have to withhold the tax
In general, having P/E means having an office in Thailand or sending staff to work in Thailand for more than 180 days a year, but it still depends on DTA agreement between Thailand and the other country.
Currently there are 52 countries that have signed DTA agreements with Thailand, including Canada, USA, Australia, China, Hong Kong, India, Japan, Singapore, South Korea, Denmark, Finland, France, Germany, the Netherlands, Norway, Sweden, Switzerland, and the UK.
I recommend that you check the DTA of your supplier's country and his P/E status before arranging any payments, or consult your nearest tax expert.