Preventing or deleting sequential invoice numbering for zero invoices

Hello,

According to our law, it is not allowed to create 0.0 invoices. Since the invoice (we are considering orders aka proforma invoices until unpaid) 0.0 invoices should not get sequential invoice numbering for the paid ones.
Is there a way using hooks or others way to prevent this happening?
The best would be to have a settings to use the "Next Paid Invoice Number" only if > 0.
I love whmcs and I don't want to search around just for this little (but still important) stuff.

Thanks, Giuseppe