In some special VAT regimes—such as Antiques (REBU) — VAT is charged only on the profit margin, not on the full selling price.
When this happens, the normal formula unit_amount × quantity doesn’t match the taxable base, so you must specify that base explicitly with the tax_base field.
Let's take this example:
| Purchase price | 400€ | What you paid for the item |
| Selling price | 600€ | What the customer pays |
| Profit margin | 200€ | 600 - 400 = 200 |
| VAT rate | 21% | VAT rate |
| VAT due | 42€ | 21% of 200 = 42 |
tax_base to report | 200€ | Margin subject to VAT |
How does this look like in the SIGN ES API?
full_amount = (unit_amount × quantity) + VAT
Here, VAT is 21 % of the margin (€200), not of the selling price.
Example payload:
"content": {
"type": "SIMPLIFIED",
"number": "12",
"series": "E",
"text": "example invoice",
"full_amount": "642.00",
"items": [
{
"text": "Mueble",
"quantity": "1",
"unit_amount": "600",
"full_amount": "642.00",
"system": {
"type": "ANTIQUES",
"tax_base": "200.00",
"category": {
"type": "VAT"
}
}
}
]
}