I have a Zen Cart client using version 1.3.9h who offers free shipping worldwide on any order over $25 US. Most of the time this works, but occasionally it charges for shipping when the order qualifies for free shipping. As an example, one order was placed for
$36.40 and recieved free shipping as it should have. Less than 30 minutes later another order to the SAME EXACT CUSTOMER AND ADDRESS for $26.65 was charged a shipping fee.

Does anyone have any idea what could be causing this?