Pirate Ship is not a shopping cart plugin. We're simply the cheapest way to buy a shipping label after you have a completed order. We do not display shipping costs on your website to your potential customers.
We recommend building shipping costs into the price of your products. As small business owners ourselves, we saw higher conversion rates if we offered "free" shipping. Another way of calculating shipping costs is to figure out the average cost for shipping an item and add a fixed shipping cost based on that average.