Zen Cart Quantity Discounts (native) on Listing Page
The native Zen Cart quantity discounts block on the product info page looks like this:
This block can also be displayed on the product listing page.
I have developed a number of these kinds of modifications over the years, and this page is designed to show you what's possible for your store.
Relevance: Zen Cart™ 1.5.x
Cost: Based on your requirements, starting at $300. This is not a plugin; it's custom software developed for your needs to your specifications. Before I can give you a final estimate, we need to discuss your needs.
Want something similar for your store? Great! Contact me with your requirements.

Example on Order Form style listing page
This is an industrial supply store, which uses the native Zen Cart quantity discount system.
Example on Image focused listing page
This is a restaurant supply store, which uses the native Zen Cart quantity discount system.