Wrong item sent is a seller-fault return reason, so this comes back for 7 days whatever the store's returns policy says, and the postage is theirs rather than yours.
It is also the easiest of all of these to resolve, because there is nothing to argue about.
Do this first
Photograph what arrived, with the packaging. Before you do anything else and before you send anything back.
The parcel, the label, the item, and the listing on screen if you can. This is thirty seconds of work that makes the whole thing straightforward.
Then message the seller
With the photograph and one sentence: "This is order #— and a different item has arrived. Photo attached."
Most of these are a mix-up in a batch of parcels, and a seller who ships forty orders after a show has very occasionally swapped two labels. They will usually know immediately which order yours was confused with.
What to ask for
The right item sent, and theirs collected or returned at their cost. That is the normal resolution.
Do not post anything back until you have agreed how. A seller may want to send a courier, or may ask you to post it with the cost refunded — either is fine, but agree it first so you are not out of pocket.
If somebody else's item is expensive
Say so and keep it safe. Do not use it, and do not post it back casually — a valuable item returned untracked becomes a second problem.
If the seller is slow and you are holding something that is not yours, that is worth raising rather than sitting on. The seller is not replying
If you would rather just keep it
That is between you and the seller, and it is sometimes the easiest outcome for both — a partial refund reflecting the difference, and nobody posts anything. Ask; some sellers prefer it to two more parcels.
If they do not respond or refuse
Raise it from the order with your photographs, choosing Wrong item sent. The seller has 48 hours to answer a return before you can ask Metzee to step in, and a dispute is decided within 48 hours of being opened.
This is a case where the evidence is usually unambiguous, so it does not tend to be a long argument. How to raise a problem
What you should not accept
Paying return postage for somebody else's mistake.
Being told to resell it. Not your problem to solve.
A partial refund that leaves you without the thing you bought — unless you genuinely want that.
For sellers reading this
Check the packing slip against the item at the point of sealing the box, not before. After a show, with forty parcels, that one habit prevents this entirely. How to pack an order
More: how to return something, something is missing from what I bought, how refunds work.