What term describes the reduction in the total bill or invoice when a minimum quantity of merchandise has been ordered?

The term that describes the reduction in the total bill or invoice when a minimum quantity of merchandise has been ordered is "quantity discount." This term specifically refers to pricing strategies where buyers receive a lower unit price for purchasing larger quantities, effectively incentivizing bulk purchases.

Quantity discounts are often used by sellers to encourage customers to buy more than they might typically need, thereby increasing overall sales volume. By offering a discount based on the quantity ordered, sellers can move inventory faster and improve customer loyalty through perceived savings. This method is commonly seen in wholesale and retail environments, where pricing structures are adjusted to reflect the benefits of larger purchases.

While the other terms may relate to pricing strategies, they do not specifically denote the structure of price reductions based on the minimum quantity ordered. For instance, a rebate typically refers to a partial refund after a purchase, bulk pricing may imply a general pricing strategy without specifying quantity requirements, and a special offer often includes promotional discounts that may not relate directly to quantity criteria. Thus, "quantity discount" accurately captures the essence of the pricing strategy described in the question.