What makes a tax invoice a tax invoice

Two different documents

A receipt says money changed hands. A tax invoice says money changed hands and carries the specific information a GST-registered buyer needs to claim the GST portion back. Missing pieces mean they cannot claim, which means they come back to you.

The word on the tax line

Plenty of checkout tools label that line “VAT”. VAT is a European tax and does not exist in New Zealand law, so an invoice claiming to charge it is describing something that did not happen.

In practice an accountant corrects it and moves on. But it is a small recurring cost to someone’s attention, and it tells any New Zealander reading it exactly where the software was built.

Ours says GST, because that is what it is.

Numbering, and why gaps are annoying

Invoice numbers should run in sequence without holes. A missing number invites the obvious question — what was invoice 47, and where did it go? — and the honest answer is usually boring, but you still have to give it.

Sequential numbering is one of those things that costs nothing if the software does it from the first sale, and is unfixable afterwards. It is not something you can retrofit onto eighteen months of orders.

None of this is difficult

That is rather the point. Every item on the list is knowable at the moment of sale, and any checkout could put them on the document. Most do not, because they were built somewhere the rules are different and New Zealand was a configuration option added afterwards.