Zen Cart Big Spender allows you to offer product or category discounts (or free gifts) based on the number of dollars spent. Categories and products can be included or excluded, choices of products or categories may be specified, discounts can be run one or many times, and many features for promoting the discounts are included.
Big Spender is configured using a series of PHP commands in a special purpose language designed for creating discounts. Because of this, while it is powerful and completely configurable, it is really intended for developers customizing sites for their clients, not for store owners with no programming background. If you are a storeowner, Discount Spender may be a better option for you.