Why the VAT number matters
An order becomes a B2B e-invoice when the buyer's VAT number is on it. Without one, the app treats the order as a consumer sale: in Belgium that means no Peppol invoice is due (you can still email a PDF invoice, see Settings); in Germany the XRechnung is issued without the buyer's VAT number.
Every number the app finds is checked against VIES, the EU registry, and the check is stored as audit evidence for zero-rated intra-EU supplies.
Easiest: Shopify's own checkout field
If your store uses Shopify Tax for the EU, you don't need to touch your theme. In the Shopify admin go to Settings → Checkout → Customer information and set "Company VAT number" to Optional.
Business buyers then enter their VAT number in the checkout. Shopify validates it on the spot, saves it on the customer's profile and, on eligible cross-border EU orders, applies the reverse charge. Invocourier reads that number from the order and uses it ahead of anything else.
Shopify's requirements: Shopify Tax active for the EU, a fulfilment location in the EU, and the field set to Optional. Not available in B2B (company) checkouts, where the company's tax ID is used instead (see below).
Alternative: a VAT field in your cart
Stores without Shopify Tax can add a cart attribute. In Online Store → Themes → Customize, open the Cart template and add a Custom liquid block where you want the field (usually above the checkout button):
<div style="margin:10px 0">
<label for="vat-number">VAT number (businesses only)</label>
<input id="vat-number" type="text" name="attributes[VAT number]"
value="{{ cart.attributes['VAT number'] }}"
placeholder="e.g. BE0123456789" form="cart">
</div>The label can say anything; what matters is the name="attributes[...]" key. The app recognizes the common names in every EU language (VAT number, TVA, BTW, USt-IdNr, Partita IVA, NIF, CUI...) and free-form variants such as "Your EU VAT ID" or "Numéro de TVA intracommunautaire". Buyers may type the number with or without the country prefix, spaces or dots. French buyers can also give a SIREN or SIRET in an attribute of that name.
My field has a different name
Open the app's Settings, section Business buyers & consumer orders. It lists the attribute names seen on your recent orders (names only, never what buyers typed) and marks each as recognized or not; a name that carried VAT-looking values is highlighted. Click Use this name and save. From then on that attribute is your VAT field.
Home also warns you when orders carried VAT-looking values under a name the app doesn't recognize.
Orders that were treated as consumer orders
Orders skipped before you fixed the field can be re-checked. Open Import past orders, tick Show skipped orders, select the orders you want and import them. Each one is evaluated again exactly like a new order: if a VAT number is found now, its invoice is issued; nothing happens automatically.
Shopify Plus B2B checkouts
Company profiles carry a tax ID per location. Invocourier reads it from the order automatically and prefers it over the checkout field and cart attributes. Nothing to configure.
VAT unverified, VAT not registered
If VIES was unreachable when an invoice was issued, the invoice goes out as B2B with the number marked VAT unverified and the conservative tax treatment; the app re-checks it on later visits and clears the mark once VIES confirms it. If VIES reports the number as not registered, the invoice is flagged and Home shows a notice. An issued document cannot be changed: check the buyer and issue a credit note or a corrected document if needed.