First - after 2 CREATE, your invoice may have more than one line item in it, so your line 3 (INSERT) won't know which line item to apply the fee to, Also, you need to specify which attribute in LineItems will hold the fees (for this example I'll assume it LineItems.Fee so your logic needs a bit of refining.
Secondly, Do you want each line item on 1 invoice to have the same fee or do you want to change the fee according to the item you are invoicing. The way you were heading with your rule, you would end up with the same fee for each line item which may not be what you want.
Please provide more detailed information so we can help you better