osCommerce Table Discounts allows you to offer quantity discounts with multiple discounting schedules depending on category, product or manufacturer. 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.
Table Discounts is configured using PHP, which allows you to specify the product, category or manufacturer to be discounted, along with the discount schedule. Many examples are provided in the help file.