osCommerce: need to fix flat rate shipping issue

on my website there is an option for flat rate shipping. you check mark the box (in the admin side), set the rate, and it will over write our usual shipping methods (ups/usps) and use the flat rate fee instead. right now however when i set the value to $0.00 (for free flat rate shipping), there is no shipping option given and the customer cannot checkout. if i set it to $0.01, then the flat rate option is shown and the customer can checkout. if u want me to choose you explain what the problem is. don't just give me the general "i can do it".