Zen Cart Addon needs to be fixed

This is a very small project... I hired someone here to make, but he hasn't been able to come through to actually make it work.

My Zen Cart store has my items, and a consignment section... the addon I hired him to make would recognise if the buyer had an item from any section + 1 item from consignment in their cart and add a $5 charge to their cart as a mixed cart fee.

The coder created the addon and it works that the notification appears in your shopping cart, but the $5 fee is not added or applied to the cart... so it does not work.

I asked him to make it work many times, and just doesn't get it working for some reason, so I am looking for someone else to fix it.