THAI ACCOUNTANT gets this question very often.

There are vendors in Thailand (or outside Thailand) that will not let your Thai company withhold taxes from them saying that their prices do not include taxes. Well, the Thai tax law makes it your duty to withhold some taxes when you pay for certain things. If you do not withhold the tax, you will have to still submit it to the Thai Revenue Department using your own money.

THAI ACCOUNTANT has 3 choices for you to consider:
1. Withhold taxes from those vendors anyway. If any of them do not let you, then you may want to find other vendors to work with.
2. You may choose to submit the taxes on behalf of the vendors (because you did not withhold from the vendors). The taxes you pay on behalf of the vendors who are juristic persons (companies and registered partnerships) will become an add-back expense, which you cannot use to lower your corporate income tax liability. Only the taxes paid on behalf of individuals can be tax deductible expense.
3. You may choose to let your vendors increase their prices by the amount of tax that you have to withhold. This way, the tax you withhold is officially not paid by you. So the tax amount will not be an add-back expense.

THAI ACCOUNTANT prefers choice 1 because it is the law. They need to let you withhold the tax from the payment and hopefully you are in the field where you do not have to deal with the vendors who do not want to comply with the law.

Good Thai accountants will deal with those tough vendors for you.