1. A method of using an apparatus for card activity-based mortgage crediting, the method including the steps of: providing a digital computer operably associated with an input device and with an output device; associating, by said digital computer, data input at said input device, the data representing card activity with further input data representing a mortgage of a property, the mortgage including principal and interest; determining, by said digital computer, a card activity reward by calculating a function responsive to the card activity of a cardholder; receiving an allocation instruction from the cardholder, the allocation instruction selected from among a group presented to the cardholder, the group comprising allocating the card activity reward to the interest of the mortgage, allocating the card activity reward to the principle of the mortgage, and allocating the card activity reward to a combination of the interest and the principle of the mortgage; crediting, by said digital computer, the card activity reward to a payment of the mortgage according to the allocation instruction of the cardholder, wherein if the allocation instruction corresponds during a year to crediting of the reward to the mortgage interest or to crediting of the reward to the combination, crediting the reward to produce a mortgage interest tax deduction for the year; and generating, by said digital computer at said output device, output including the card activity reward being credited according to the allocation instruction of the cardholder.