I'm new to ZC, so if this is simple or there's an easier way to do this, I do apologize. Here's my issue. I set up an observer to modify the cart price. Here's what I check: First if the customer came to the site via a specific URL (I set a session variable to hold if they did or did not). Then, if they did and they have at least 3 items in their cart and 1 of the items is a specific item (say #100) then the price of that item is subtracted. It shows up correctly in the cart. It was resetting back in step 2 of checkout, so I modified it to reduce the global $order->total by the product amount. After that it shows up fine in step 2. After a customer enters their credit card info and it gets authorized, the final confirmation screen (step 3) shows the full price again. I can't figure out how to make this work. Is there an easier way to:

1. follow a link
2. buy any 2 items + item 'A' and get 'A' for free.

Any help is appreciated. This is a holiday promo and would like to get it fixed!