Hi, I know it's an old topic but I would like to add.
This goes for Denmark too!
It is not legal to use order number as the invoice number!!!
Order is one thing, Invoice an other thing.
Customer makes an order. You can say, that is a request to buy something. The seller has the option to cancel the order, could be that product is not available any longer, payment on credit card did not get accepted, etc.
When seller is ready to ship order, then he needs to send an Invoice. That's generated when it's official an income of money, if you ask an Accountant. Also, in Denmark you are not allowed to charge the creditcard before you have actually shipped the product. So when you ship and charge customer, then you generate an Invoice - and this has to have it's OWN number!
Best regards,
Henrik