This isn’t a problem, Onyx will create a Cash Account that you can use for all invoices, this means that you don't then need to create any customer accounts (though please see below advice). If you use the cash account when creating an invoice, you can enter specific details by pressing the 'edit' button.
Please note, if you do this you will not be able to use many of the advanced customer management features such as mail shots, emails, SMS, MOT and Service reminders etc.
Please be very careful before deciding to go down this route, in our experience first time users (especially those moving to a computer based system) find creating accounts a bit of an alien concept (not to mention time consuming), however, once an account is set up you need never do anything with it again, Onyx will take care of selecting the correct account for you when you enter the registration number. By having accounts you can set up credit terms, check customer turnover, send promotional letters and reminders, see where your customers live so that you can target new areas with leaflet drops (for example).
See also, Rapid Invoice