invoice, cost of goods sold, process payment question

[John Griessen]

Hi,

How do I connect cost of goods sold to an inventory item sold
on an invoice line?   Choices in creating the invoice seem to be
income accounts called assets:inventory  and sales:services and then at the end 
is just the heading Liabilities with no detail and no Expenses which has COGS in it.

Is there a way to add to the choices available in the invoice creation, or is 
there another way to use COGS?

John Griessen