If you sell a customer a product or a service, you need to give them an invoice (bill) by law if both you and the customer are registered for VAT (a business to business transaction). An invoice is not the same as a receipt, which is an acknowledgement of payment.
The invoice must include certain information such as:
- how much the customer needs to pay you
- when the customer must pay you
You and the customer also have certain obligations on payment.