Nick here from Shopify. Great question!
While Shopify does have a discount feature for buy X get Y free, it doesn't have the aspect of it increasing automatically which you are looking to do. You can see the Shopify guide for this here.
I'm thinking it could be worth your time exploring some apps in the Shopify App Store which would have more features for this and could hopefully do what you want and need it to do. Let's take a look at some of these below:
If none of the above apps can automatically apply the discount as it goes higher, your best next bet would be to have something custom made by a Shopify Expert. Shopify Experts know the platform inside and out. While this would be a higher upfront cost, it wouldn't be a monthly payment like some of the apps. You can see the full Expert directory here.
Do you think any of the above would work for you?
All the best, Nick
Did you end up finding an app that meets your needs?
In Shopify you can create a discount like Buy 6 Get 3 free by selecting Buy 2 Get 1 option and later on you can take a help of an app which creates Automatic Discounts so i will refer "Kart Discount App" for that Stack and Automatic Discounts. Now you can get rid of this issue.
Am also sharing the app link for your reference i.e; https://apps.shopify.com/discount-coupon-field-in-cart-page
You can offer your customers discounts that apply automatically at checkout and on cart. You can create percentage, fixed amount, or buy X get Y automatic discounts.
For an automatic discount to apply correctly, your customers need to add all eligible items to their carts before they move to checkout. This includes the items that they need to buy to qualify for the discount as well as the items that they get as part of the promotion.
You can have only one active automatic discount at any given time. Automatic discounts take precedence over discount codes, and customers can't apply multiple discounts to a single order. Discount codes are unavailable for customers who have an automatic discount applied to their . Someone referred a app link on shopify were you can have Automatic Discount and Stack discount .
With the help of discount field on cart page which leads to more conversion regarding the details i have found one app in shopify app store I.e; KARTDISCOUNT
1)Show Discount Field on Cart Page and Side Drawer.
2)Stack Multiple Discount on Single Order. Example: 1 Automatic Discount + 1 Coupon code discount Example: 5% off on jeans, and 10% off on shoe if customer has both jeans and shoe in cart.
3)Discount Combination: Generate combined discount codes, that will be equal to applying 2 discount codes in stack, just like the above point. However, you can give away only one code in your marketing campaigns. When your customers apply that code, they get 2 benefits same like theabove example.
4)Automatic Discount: Add multiple automatic discounts, enabled at the same time unlike Shopify's automatic which allows only 1 automatic discount as a time.
Try this ,you can get rid of these problem.