An electronic transaction, in which a payment amount is deducted from a customer account and added to a provider account, is prepared in cooperation between an account computer and a browser-enabled customer computer. The account computer forwards invoice and credit information as well as calculation instructions in portions of a page to the customer computer. The customer computer interprets a first page portion with a browser to present different invoice items and credit items on a screen. A user selection of the invoice and credit items is received and then the payment amount is calculated by offsetting the amounts of the invoice and credit items with instructions in a second page portion. Thereafter, a response with the payment amount is returned to the account computer.