OK, I've added a screenshot and I think there's a bug in MarketPress.
I have an order for over $100 and over 7lbs of weight. We have two shipping methods, Media Mail and Priority Mail and the location of the customer is in Georgia US. I use the shipping method of calculation based on weight and this order came through with a shipping cost of .50 cents.
Based on other calculations of other orders and their accuracy I had to dig into why this order was different.
One of the products on the list contained an 'Extra Shipping Cost' of .50 in the product set-up. It appears that this field became the entire shipping cost and not an extra shipping and with this field filled in, it didn't calculate shipping for any of the other products.
This appears to be a bug. Attached is the screenshot of the order.