All invoices and bills must have a unique and verifiable invoice number that identifies them.
In order for an invoice to be a valid sales document, it must have an invoice number. It cannot be any number; there are strict rules for how to use the invoice number.
Sales documents must be pre-numbered on pinted forms or by machine-assigned numbers with a verifiable sequence, or be marked in another way so that complete registration of issued sales can be easily verified.
Requirements for unbroken number sequence with invoice numbers
In order to track all the invoices you send out, they must have a unique number in an unbroken number sequence. The first time you create an invoice, you can start with any number you want, but then you must continue creating the next invoice with the next number, and so on. Many choose to start with 1 or 1001. Then the invoices continue with 1002, 1003, 1004, and as far as you plan to invoice.
If you switch to a new invoicing program, the first invoice in the new program should continue the number sequence from the old one. So, if the last invoice you sent had the number 1089, you should start with number 109 in the new system.
Because the number sequence must be unbroken, you can not delete an invoice. It is also illegal to change it after it has been issued. If you have sent an invoice that contains errors, you must therefore create a credit note to nullify it.
Use an approved invoicing program
The invoice number must be either machine-generated or pre-printed to be valid. The easiest way to ensure that it is correct is to use an approved invoicing program.
In an invoicing program like Conta, the invoice number is automatically generated every time you create an invoice or credit note, so you don’t have to worry about if your invoices comply with the rules and meet all requirements.
Alternatives to invoicing programs
Many wonder if they can use an invoice template or create their own invoice in Word. The answer is simply no. It is actually illegal because the invoice numbers are not pre-printed and because it allows for editing the invoice after it has been sent out.
The only legal alternative to an invoicing program is to print it on pre-numbered invoice forms like they did in the old days.