I am currently working on my first oscommerce install. I am currently using the paypal_ipn module. I am also currently running into this exact same issue. When my customer gets redirected to paypal there is one item which is being "purchased" for a total of $0. It makes me glad to hear that I am not the very first person to run into this issue.
As a result I am curious to know if you have come across a resolution or if anyone else has for that matter.