Have you watched the Invoicing video yet?
Especially the extended cut.  

What exactly is your formula to set the price and how are you looking it up to set it on the order from?

Hi & thanks for your reply.
Yes, I watched the extended cut invoice when I tried to enter the product I got the error message
"Run Time error 3164" which on help says 'The field cannot be updated because another user or process has locked the corresponding record or table'

This is what I do not understand?

OK show and tell time.  
We need to see some screenshots of your forms and queries in order to help walk you through the problem.  At this point I'd just be guessing about what the issue is.
Feel free to use dummy data or obscure sensitive data.  Design view is helpful to check out your formulas.

Hi again, Thanks for taking a look at this. Everything seamed to work ok until I tried to add the details of the products to the order form.

Hi & thanks for your help. I worked through the problem & solved it. So please don't spend any more time on this.

Gordon good to know.  Sorry I got side tracked with work and didn't help out quicker.

Scott, No problem, the fact that I could ask was great. I now understand what I was doing wrong!