Thanks for reply but not a solution to my issue.

I understand adding custom field to invoices and do a one, Purchase Order Number.

However I was wanting to add a Custom Field to a Customers Details and print this on each Invoice and Deliver note. The way each time  we invoice a customer the details are automatically added to the Invoice & Deliver note. 

The reason is I have over 700 customers, and about 20 we need to ship via a different method.

Rather than tying to remember who the 20 special customers are, would want a field in the customers details to help identify them when I raise and invoice for them.