This feature of the Promotion code record only applies to webstore items applied to a Percentage Rate type of Promo code record.
There is no strikeout effect for Flat Rate type of promo codes as the discount is applied to the whole order.
The setup of the promo code record is as follows:
1. Navigate to Lists > Marketing > Promotions > New > Standard Promotion.
2. Input the required fields like Promotion Code name and Discount.
3. Check DISPLAY LINE DISCOUNTS field.
4. Set the Rate type radio button to % and enter the discount value.
5. Under Codes subtab > enter Coupon Code value.
6. Click the Items subtab and assign the desired web store items that the promo code will apply to.
7. Save the changes.
When webstore shoppers add the qualifying items to the cart and apply the promo code, the strike out effect would be seen.