Hope you are doing well.
You can achieve the same using iCart upsell app. You have to select the discount type = Fixed amount per item as shown in the image below.
Hope this will help you!
There are apps known as Quantity Break Discounts that work by offering tiered discounts (flat rate, percentage, etc.) based on the volume of the product that is ordered. This should accomplish exactly what you are looking for! I've compiled a few great app options from the Shopify app store below:
Check these options out to see which one is the right fit for you! What type of business do you run by the way? Do you find that your business relies on high-volume product orders to succeed?