partial/down payment with authorize.net AIM?

Can I set the Zen Cart AIM payment to authorize (to authorize the full amount through authorize.net) and then capture only part of the payment in the authorize.net web interface as a down payment? Then when the product is shipped, I want to capture the rest of the payment.

In summary, can I authorize through AIM then capture two partial payments through the authorize.net web interface?

I know there is an addon that lets you process this offline, but I would like to do it all online. I also know from the FAQ that you can authorize first and capture later, but it doesn't say anything about partial payment/down payment.

The authorize.net gateway only allows you to capture funds once per transaction. So, the simple short answer to your question is no.

The long answer is that you might be able to accomplish something similar by forcing all such transactions into a recurring billing profile and capture the first payment immediately with the subsequent payment(s) to follow at a later, scheduled, date. However, recurring-billing support is not built-in to Zen Cart at the present time.