Please how do you post cost of sales of non inventory item. A one off transaction
A non inventory item doesn’t have a cost of sales - only inventory items have cost of sales.
Your probably right for the non inventory item, although I have seen systems handling that (Cost of sales) for non stock items as a percentage of sales.
One way to handle this would be to have expense accounts specifically associated with non-inventory sales. For example, if you sell non-inventory inspection services, you could have expense accounts for inspection service marketing, inspection service vehicles, etc.
Or you could use tracking codes and treat those portions of the business that handle non-inventory sales as separate divisions.
But it is very difficult to capture all expenses associated with a specific sale of a non-inventory item. Even with inventory items, accounting systems don’t typically try to incorporate all that into cost of goods sold. That’s where the terminology cost of goods comes from. It refers to the cost of the goods themselves, not the cost of doing business to sell the goods. Those are typically classified as operating expenses.