How does the $ off cart discount work for customers?

When the coupon code is applied (either automatically or manually by the customer), it will take a discount of $20 (for example) off the total purchase amount. Post-purchase, the price is displayed as a pro-rata discount distributed across all eligible items in the shopping cart on the purchase invoice.

For example, if the total is $160, the discount amount would be 20/160 = 12.5% off the cost of each item.