Abakira Quantity Breaks

Quantity breaks on your product pages — buy more, save more — with the discount applied by Shopify itself, not by a script that rewrites your prices.

See the exact amounts before you publish. The app previews what a buyer will really pay, Shopify’s rounding included, on the price of the products you target.
It refuses what it cannot get right. A discount set in money cannot become one percentage across items priced differently, so the app says so instead of publishing something you never intended.
An end date really ends the offer. Shopify enforces it. A paused or expired offer stops applying at checkout, not just on screen.
You can see what it earned. The app matches orders back to the offer that produced them, and stores no buyer personal data to do it.

Install it from the Shopify App Store, then open it from Apps in your Shopify admin.