Working With The authorize.net Server Integration Method (SIM) Payment Gateway, Part 1
Example 1: A Form That Computes Order Total Via JavaScript

Make selections, then submit the form to see the value of the order_total_input hidden form field.

Choose a basic membership:

Annual banquet tickets:

Would you like to purchase a T-shirt?

Your order total: $0.00

Example 2: Poisoning the order_total_input Variable
Example 3: Using JavaScript To Change Individual Order Options Blog entry: Working With The authorize.net Server Integration Method (SIM) Payment Gateway, Part 1: Don't Use JavaScript