Dear narinder_garg,
I do understand your disappointment as the order once placed can not be cancelled and discount coupon can not applied now. At the time of payment checkout you can see a field where you need to enter discount coupon/ promotion code and click on apply button to avail the discount. Kindly note that products listed under special deals e.g., deals of the weeks are exempted from any discount / promotion as they're listed on discounted price. You can use the coupon code for your next purchase.
@arrajiv : eBay coupon codes are 10-digits alphanumeric one. Kindly elaborate your concern. I would be happy to assist you! 🙂