You post your own parcels, so you decide what postage costs. What you are describing is not the price of one item but the price of a box: what it costs to send the first thing in it, and what each further thing adds.
A rate is three numbers, not one
Each rate you create answers three questions. What does the first item in a parcel cost to post? What does each additional item add to that same parcel? And what is the most one parcel can ever cost, however much somebody buys?
For anything that goes in a single mailer — cards, prints, small flat goods — the second number is usually zero. Ten cards fit in the envelope one card fits in, so the buyer pays for the envelope once and every card after the first adds nothing. For items that each need their own box, the additional amount is close to the first, because you really are buying a second label.
The ceiling is not decoration. An additional amount of fifty cents looks harmless until somebody buys thirty things and is charged fifteen euros of postage for a parcel that cost you five. Set the ceiling to the most you would ever actually spend sending one box to one address.
Most sellers need two or three rates
Rates describe how you pack, not what an individual item is, so they are attached to products rather than typed out again each time. A seller of trading cards and sealed boxes needs exactly two: one where everything travels together, and one where each item is its own parcel.
You can create a rate while you are writing a listing, so your first product does not send you somewhere else to set one up. A rate you have finished with is retired rather than deleted, and only once nothing live is listed on it, so items you have already sold keep the terms they were sold under.
Several purchases become one parcel
When the same buyer takes a second item from you, it joins the parcel already open for them rather than starting a new one. They are charged the difference their new item makes to that parcel, which for a zero additional amount is nothing at all. This is what stops a buyer taking ten singles and being charged ten lots of postage.
The parcel is not tied to one show. It stays open for three days from the first item in it, so a buyer who comes back to a second broadcast the following evening is still filling the same box. It is tied to one address: if they change where an order is going, the next purchase starts a new parcel, because it is genuinely a different box going to a different door.
Different sellers are always different parcels. A buyer taking items from three sellers in one show pays three lots of postage, because three separate people are packing three separate boxes.
You do not have to wait three days
Three days is when a parcel closes by itself, not how long you have to hold somebody's goods. If you are packing tonight, close the parcel and post it.
Closing a parcel does end the bundling for it. Anything that buyer takes afterwards opens a new parcel and is charged a fresh first-item rate. That is not a penalty — they see the real price before they commit to anything — but it is worth knowing that a buyer who is clearly still shopping may be worth waiting an hour for.
If you close one too early and then wish you had not, you can still combine parcels for the same buyer by hand before either has a label. Combining or splitting never changes what the buyer paid; the postage difference goes on your ledger, because you are the one who chose the arrangement.
The difference is yours, in both directions
You set the rate and you buy the label, so the gap between what you collected and what the post office actually charged is yours. When a bundled parcel costs less than the separate ones it replaced, you keep the saving. When you have under-priced a rate, you cover the shortfall.
This is worth a moment of arithmetic when you first set your rates, and almost no thought afterwards. Weigh a typical parcel, check what your carrier charges for it, and set the first-item amount at or slightly above that. Most sellers get this right once and never revisit it.
If a buyer cancels an item, they are refunded what the parcel stops costing rather than the line they happened to be charged. If they cancel the item that was carrying the parcel's rate and something else is still coming, they keep paying for the box that is still being sent — you are not left posting a parcel you collected nothing for.