Have you loaded the old Quantity Discounts mod by swguy? You may have to go back to version 1.12.1 in order to work with this old version of Zen Cart.

As an option, using the options you've already tried, you will need to set an actual price and edit it each time the basic price changes. The old method did not support percentages.

If the boxes are $10 each, a 5% discount on 5 boxes would be a price of $47.50, and 10 boxes would be $95.

Ask your client if they have insurance on their home or auto. They may need some for their cart, as vulnerable as it is. They may even be violating federal law depending on the payment methods being used. That version will be 24 years old this year.