1. A patronage incentive system, comprising: a computer system for interactive communication between a plurality of enrolled customers having access to the Internet and at least one participating retailer having at least one point-of-sale terminal, said enrolled customers purchasing a product offered by said at least one participating retailer at said at least one point-of-sale terminal thereof at a retail price, a saving software program integrated into said computer system at said at least one point-of-sale terminal of said at least one participating retailer; a predetermined single mutual fund; each of said customers initiating the enrollment thereof in said patronage incentive system by interacting with said saving software program through the Internet to establish a personal account associated with said each customer and to receive a predetermined identifier of said each customer; said saving software program for: (a) rounding up said retail price of the purchased product to a rounded price amount, (b) crediting the difference between said rounded price amount and said retail price to said personal account associated with said each enrolled customer making purchase, and (c) transferring the amounts credited to said personal accounts for said plurality of enrolled customers into a merging account for investment into said predetermined single mutual fund, said merging account containing combined differences between said rounded price amounts and said retail prices for said plurality of enrolled customers, whereby said merging account and said single mutual fund are shared by said plurality of enrolled customers.