Shipping not Calculated Correctly - Inconsistent!

OK, so the majority of our orders come through without an issue but once in a while the shipping is calculated as $0.00 and this has happened twice. Nothing was common about the two orders so I thought it might just be a glitch.

Last night we had another order and the shipping was calculated at $1.00 which was an order that should have been about $40.00 in shipping.

These are the only three orders that have come through this way, every other order is correct in calculating the correct shipping using USPS weight based shipping.

Since the launch of our site we've had 64 orders through Marketpress and 3 have been incorrect when it comes to shipping.

Is there a known bug?