The supplier asks for your VAT number so that it can invoice a delivery to a company in another EU state without VAT. Without a valid number, the sale may be treated as one with Romanian VAT, and the supplier may be obliged to charge it.
You can check the number yourself in the European Commission's VIES system and send the supplier your exact company name and address as they appear in the registration. The details on the order must match. For your specific case, especially mixed deliveries or pickup with your own carrier, ask your accountant to confirm.
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