Electronic financial transaction with balancing invoice and credit items via page

A computer system for preparing an electronic financial transaction, the system comprising a first computer of a provider, a second computer of an account institution, and a third computer of a customer operated by a user, wherein the first computer, prior to the preparation of the transaction, forwards invoice and credit information to the second computer, and wherein, after preparation of the transaction, the second computer performs the transaction by deducting a payment amount from a customer account and adding the payment amount to a provider account, the system further comprising:

• means for sending a page to the third computer to forward invoice and credit information in a first portion of the page and forward calculation instructions in a second portion of the page;
• means for interpreting the first portion with the third computer to present different invoice items and credit items on a screen;
• means for receiving from a user of the third computer a selection of at least one invoice item and of at least one credit item;
• means for calculating the payment amount by offsetting the amounts of the invoice and credit items in accordance with the instructions in the second portion;
• and
• means for returning a response with the payment amount to the second computer.