Cost of sales

No,
cost of sales= opening stock + purchase - closing stock.
It is the cost that related directly to company stock. Therefore, it is deducted directly from the sales, so that you can calculate the gross profit.

Sales commission is a kind of operation expense, therefore, it should be deducted after gross profit.

Sales commission can be determined according to sales, for example, sales commission is 10% of sales. If the sales is HK$100,000 , then the commission will be HK$10,000. Howvever, it is still operation expense. Therefore, you should deducted after gross profit and before net profit.

No, as sales comm is a operation expense. Cost of sale just include all direct cost with the goods or services from purchases to shop only. Therefore, sales comm is after sales transaction not directly affect sales, just expenses.