Contribution per order is what a single average order leaves after the costs it caused: the goods, the parcel, the payment fee and, depending on the question, its share of advertising. It is the number that converts every other decision into orders, which is what makes it the most portable figure on a statement.
On the example store an 85 euro order leaves 44,68 euro after goods, fulfilment and fees. That one figure prices a great many questions at once: how many orders cover the month's fixed costs, how much a return costs in sales that have to happen again, whether an acquisition cost is affordable, and how much volume a discount has to bring back.
Which costs belong in it depends on the question being asked. After goods, parcel and fee it answers whether operations pay; after advertising too, it answers whether customers pay. Both are legitimate and they give different answers, so a statement should be explicit about which one a figure is.
Averages hide variance, so it is worth computing per product as well as per store. A catalogue average can be comfortable while a third of the orders contribute nothing at all.