Shipping is the price of a box, not a fee attached to each thing you buy. When you take several items from the same seller they travel together, and you pay for the parcel once.
The number under an item is what it adds
The shipping figure shown beside an item is not what that item would cost to post on its own. It is what it adds to the parcel you already have open with that seller. The first thing you buy shows the full cost of the box. The second often shows nothing, because it fits in the box you have already paid for.
That number is the number you are charged. There is no recalculation afterwards and no surprise at the end: whatever is displayed before you commit is what appears on your receipt.
One parcel per seller, for three days
Your parcel with a seller stays open for three days from the first item you buy from them. Anything else you take from that seller in that time joins it, including items bought in a completely different show on a different evening.
Sellers pack their own parcels, so each seller is a separate box. Buying from three sellers in one show means three parcels and three shipping charges, however small the items are. That is not a fee — it is three different people going to the post office.
The parcel is tied to one delivery address. If you change where your orders are going part way through, the next thing you buy starts a new parcel, because it is a second box going somewhere else.
Sellers can post before the three days are up
A seller who is ready to post does not have to wait for your parcel to close on its own, and it is usually better for you that they do not — it is the difference between your order arriving on Tuesday and arriving on Friday.
If a seller has already sent your parcel and you buy from them again, the new item starts a fresh parcel and shows a fresh shipping cost. You will see that cost before you buy, in the same place you always see it. Nothing is added afterwards.
If something is cancelled
When an item is cancelled you are refunded what the parcel stops costing, which is not always the exact figure shown against that line. If you bought a boxed item and then a small one that shipped free alongside it, cancelling the boxed item leaves the small one still on its way — and a parcel is still being sent to you, so you keep paying for that parcel.
The result is the same either way round: you end up having paid for the box that actually reaches your door, and nothing for the one that does not. If a refund does not look right to you, contact the seller through the order rather than outside it, so there is a record attached to the purchase.