Big Chooser for osCommerce is product based discounting. Buy multiple items, get a discount on other products or combinations of products, or receive other products or combinations of products free. Note that in many cases, Paypal does not interoperate correctly with any osCommerce discounting modifications (including mine) by default; changes are required. Please review My osCommerce Payment Integration Notes and test with a free module prior to ordering a paid module.
Big Chooser 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 Chooser may be a better option for you.