Why is my custom discount code not working on the checkout page?

Shopify Discussion
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Topic summary

Issue: A quantity-based “BUNDLE DISCOUNT” code that previously worked now fails at checkout. The discount appears momentarily, then is overridden and disappears.

Setup: The code applies tiers by pairs purchased (3=5%, 6=10%, 10=15%, etc.). The unexpected behavior began today without a stated configuration change.

Evidence: Linked screenshots show the discount applied and then removed within seconds during checkout. A checkout URL includes the parameter “checkout_type=c1,” which the poster wonders may be related; it is a URL parameter observed during checkout.

Context: A store/product link is provided for live testing. No mention of other active discounts, apps, or automatic promotions that could be conflicting.

Status: Unresolved and open. Key questions: why the discount is being overridden at checkout and whether the “checkout_type=c1” parameter affects discount application. Attachments (screenshots) are central to understanding the issue.

Summarized with AI on December 28. AI used: gpt-5.
1

We are experiencing an issue with our checkout page discount.

Details:
For the code, we have this: https://prnt.sc/XkpI69QC19qZ. We created this “BUNDLE DISCOUNT” code based on the cart items. For example, if a customer buys 3 pairs, they will get a 5% discount; if they buy 6 pairs, they will get a 10% discount; and if they buy 10 pairs, they will get a 15% discount, and so on.

Issue:
It was working properly before, but today, upon checking, it is not functioning correctly.

Observations:
During checkout, the discount is applied https://prnt.sc/oXCVAjzzQXms, but after a few seconds, it gets overridden, and the discount is removed https://prnt.sc/mynK1uvWPDdJ.

Uncertainty:
I am also unsure if the checkout_type=c1 parameter is causing the issue https://prnt.sc/DuHefZ6Nve3X.

Store Link:
https://wamaunderwear.com/products/hemp-boxer-briefs

Thank you!