Checkout allowing free shipping randomly?

For some reason it seems that certain products are giving the customer free shipping when they're not supposed to. I have free shipping on order over $100, but these products are well under $100 and still show free shipping as an option. I can't figure it out, I have not changed any settings at all and all of a sudden this is happening. This problem is limited to 2 products (that I know of) right now.
Zencart v. 1.3.8a

Are they possibly mis-marked as:
Always Free Shipping YES
Product is Virtual YES
Product Weight 0

Thanks Linda, adding a weight (formerly at 0) took care of it. But that baffles me because I have thousands of other items with 0 weight that do not allow free shipping.

Without sitting down and going through your database and testing numerous scenarios it is hard to speculate on the issue here ...

I would have suspected 0 weight on any product alone in an order causing an issue, if it were the weight doing this ...