Shipping charge not working sometimes? What could be causing this?

Client emailed me last night saying an order had come through where a shipping charge had not been applied to this item:

I went through a test purchase of the item, and the shipping charge showed up so of course I told her she was smoking crack. (clients you can joke with are the best.) She tested it on her end a couple of times, and the first time it worked fine, but the second time it didn't show up on the paypal end.

Is there anything going on with Marketpress that would be causing this? Seems like other orders are going through fine, just disappointing she had to pay outta pocket for the shipping on that order and we'd like to prevent that from happening again.