I checked with the revenue department.
From Tax’s view, Below please see the summary of the tax concern:
1. It is service agreement. It have to deduct withholding tax 3 percentage.
2. Stamp duty. Please see the notification of Director-General of the revenue department on Stamp Duty No.54.
With effect from 5 April 2015, Stamp duty is required to be paid by cash to the Revenue department instead of by affixing stamps.
Hire of work agreement with remuneration of Baht 1 million or more.
Stamp duty rate is 0.1 % of the contract value .