I think the most important thing to remember is that with most ecommerce software there are two ways of setting up the shipping calculations. One is by weight (which is what I have used on my stained glass website) but that requires that all items have an accurate packed weight in the database for use when calculating shipping. But even that method can get messed up. For example, if I have a fragile object that requires a lot of packaging to survive travel, then the packaging can add significantly to the weight. If someone buys two of them, I can place them both in the same box which will save on the weight. However, the computer does not know this, so it just adds the weights for each item together when calculating shipping.

The second way to handle shipping charges on a website is by total purchase price. This method is by far the easiest to implement because there is no need for extra information in the database. Every item already has to have a price. Since Kiltstore offeres free shipping for orders over a certain amount (if you are a member of their club) it is most likely that they are using shipping based on purchase amount. That completely disregards the item weight, and is usually priced so that the shipping costs average out over all the transactions. If this is the case, your shipping cost is most likely subsidizing the shipping of someone who ordered a heavy item but that cost the same as your order. Also, they must cover the cost of the free shipping offer, and that is probably amortized across the remaining orders.

Finally, there is the issue of handling charges. A website can either apply a handling charge as a single charge per order, or as a handling charge per item. If the charge is per item, the more items in your order (no matter how light they are) the higher your handling charges. So if this method is used, then handling charge for on extremely heavy but fragile item that requires a lot of packing will be less than the handling charges for 4 extremely light items.

I would contact them directly.