Thanks for your question, It's an interesting one. In fact, I called and talked with the tech support people at Paypal to ensure that I wasn't missing something when I gave you this answer.

The bad news: they do not support zero cost transactions in the shopping cart. Period.

I even tried creating a one-cent item with their "button builder" tool then in the HTML code changing the cost to zero, but when you try to add that item to your shopping cart the Paypal system generates a "please enter an amount greater than zero" and it won't go through.

At this point I am afraid there's no way to do this with the Paypal system. Given that, I'd suggest that you just offer "penny upgrades" or similar and have a picture of a penny right next to the item to emphasize that it's only one cent more to add those to an order.

Wish I had a better answer. I imagine if you wanted to go through the hassle of switching to a different shopping cart system you could easily add zero-cost transactions, but Paypal doesn't support that.

The only other thought I have is that perhaps you could have a pull-down option menu for the item that you know people have to purchase, and on that menu it would have all the free optional add-ons? Or even do that as check boxes associated with a specific item in your store?

Good luck however you proceed!