When you sell prescribed items, GST is not charged. You must use the GPG tax code to indicate that this is the sale of prescribed goods. This does not apply if you are selling to the end user (normal GST applies).
Note: When prescribed goods are sold, the invoice must display the special text "Sale made under customer accounting. Customer to account for GST if $xxx". The "Prescribed Goods (Singapore)" template does this, and others can be customised.
