There are a couple ways to do this. First, you could use an actual programming language, allowing you to send the form to a second page, collect all the form info, add everything up, and then charge the person internally using Authorize.net's AIM method. Second, you could have a single hidden x_amount field and use Javascript to calculate the total amount and update that field whenever they click or unclick something on the form. Note that this method obviously won't work if Javascript is turned off, so you'll need to disable form submit for anyone not running Javascript. Maybe by having the form default to sumitting to a "sorry, you don't have Javascript turned on" page, then have onsubmit change the URL before it submits.
Do you know any PHP, by any chance? Or does this need to be HTML / Javascript only?
