For example, say we have a product "CCB - coffee cups blue". COGs is $2 and normal sale price is $3. Customer A buys 20 and we add 3 promotional to the order. So we ship 23. The Invoice has CCB Qty 23 - $3 = $69. We add a line Promotional DIscount Qty 3 at $3. Net amount due is 60. 23 CCB's hit COGS so COGS records $46.

Another approach would be to create a separate part code for the promotional items. CCB-Promo. You set that up with COGs $2 and sale price $0. In that case your invoice to the customer would have Qty 20 CCB @$3 and Qty 3 CCB-Promo at $0.

Both approaches will properly record to the income and balance sheet. It depends how much detail you want to retain about your promotional expense - total expense, or total expense plus unit quantity.