I tried reproducing the problem by using the same address for myself that the customer was using. I didn't have any problems, but then again, I was trying to purchase different items, not the same one she had.

the shipping rate is $3.50 for the first item and $1.50 for each additional item. Right now it is set to put a handling fee of $2 on all orders with a shipping of $1.50 per item. That's how I get zencart to set the shipping the way I want it. I think there is only one shipping module installed. the customer was not supposed to be charged shipping in this case however.