According to Article 4 of Tax Collection Act, when a foreign diplomat in Taiwan with a diplomat identification card and tax exemption card issued by the Ministry of Foreign Affairs purchases goods or services, the vendor should issue a zero-tax-rate invoice and provide exemption from sales tax.
If the buyer of goods is a member of a foreign mission in Taiwan, the correct way for the vendor to issue the GUI is to use the actual sales price (the selling price that includes sales tax÷(1+tax rate)) and issue a zero-tax-rate GUI.