[mothered]

But if we would assume that it fails. What could happen?

Nothing, but to legitimately refund the Item, depends on your reason for the Initial return.

For Instance, If It was a change of mind (after viewing the Item), simply say that you were In a hurry and packed the Incorrect one. Then send back the original for a refund. That's why I keep asking questions - to clarify every step of the SE, thus be prepared for every unexpected event.