Every listing shows its shipping charge before you add it to the cart. A seller may set that charge to zero and offer free shipping.
How the cart calculates shipping
The cart groups items by seller so you can see who will fulfil them, but it keeps the shipping charge from each listing. Two items from the same seller can therefore have two shipping charges.
Checkout rechecks every listing and adds the shipping amounts for the items that are still available. The total shown before payment is the amount eBuy will attempt to charge.
Who chooses the amount
The seller sets the shipping amount for each listing and chooses the carrier. They cannot change it while the item is reserved or after it has an order.
Why did shipping increase when I added another item?
Shipping and buyer protection are different
The shipping charge helps the seller pay to send the parcel. Buyer protection is the order-review process used when an eBuy purchase does not arrive or is not as described. A shipping charge is not a promise that the parcel has separate courier insurance.