I need to ad the vehicles details to the invoice , like the registration no and the km reading ?

I can help you add vehicle details to an invoice in QuickBooks Online, @autofix-tzn-gmai.

You can put the registration number and kilometer reading in the Description field or on the Messages field of the invoice. Let me show you how: 

1. From the left menu, choose Sales.
2. Select Invoices.
   
   
3. Search and open the invoice.
4. Add the details to the Description or Messages field.
5. Once done, click Save.
   

Additionally, you can create a custom form to add the details. Please refer to this article for the details: Create and edit custom fields.

I've also added this article that can guide you to create the same invoice for multiple customers

 in QuickBooks Online.

Don't hesitate to click the Reply button if you have other queries. I'll gladly help you as soon as I can. Have a nice day.