I have go through last shipment and invoice show total price is SGD 10. Which definitely custom house will reject. As custom house do have market central price as to control that importer will not declared under value to much to avoid import duty tax and vat.
Thailand is not free import duty tax as Singapore so invoice value is one of the factor will effect to custom clearance procedure.
Please take note that some custom officers have right to conflicted or penalty if they feel that this is intention of avoiding import duty tax. Some officers may allowed you to amend declaration if they feel that its’ under value. Please take note of this notice