There is one discount offer in my store now. That is buy one get one 50% OFF. I am using an application for this, and this discount are displayed in Cart and Checkout so I am happy with that. Beside this, I want to give discount code in some days in week. I know there are some applications can do it at Cart section but I want to apply this code in Checkout section. Is there any ways to do it congruently and fluently.
I'm Miles from the Social Care team at Shopify. At the moment, multiple (or stacked) discounts are not possible after the cart page. As you've alluded to, there are apps in the Shopify App Store that do allow stacked discounts or bundles at the cart page, but you won't be able to find one for the checkout. The main reason for this is that Shopify doesn't allow edits to the checkout.liquid (checkout code) to ensure the security of the platform.
I'll be happy to add your feedback to our team to review so we can look at offering stacked discounts in the future. As always, there's no guarantee that this feature will become available, but it's always good to pass on the feedback just in case.
In the meantime, I recommend giving apps a chance. You can look at apps for bundle deals (where a customer can buy multiple products at a discounted rate), or even cart-based discounts. I've recommended some of my favourite apps below to help you get started.
If there's anything else I can assist with, please don't hesitate to ask.