Oscommerce tax on free shipping

I have an old highly customized Oscommerce site. It "seems" like the situation has changed whereby now, the tax is no longer charged on zero weighted products, when it should be. I would need this to be put back as such. So I would need: 1. Tax and shipping charged on weighted products. 2. Tax only charged on zero weight products. I have the Canada Post module running. It has since the beginning.