Everything here turns on one thing: has it been posted yet?
Before it ships
Message the store. Not a rule in the app so much as a courtesy that works — a store that has not packed it yet can usually change an address, swap a size, or cancel and refund you.
Be quick. A store with a 24-hour dispatch time may already be at the counter.
If they will not or cannot, ask them to cancel and refund, then buy again correctly. Clean, and the refund goes back to the method you paid with. Can I change my address?
After it ships
It is no longer a change, it is a return — and the store's own policy decides whether a change of mind qualifies.
What is not up to their policy: damaged, faulty, wrong or not as described comes back for 7 days from delivery whatever they have written. How returns work
Cancelling because you changed your mind
Before it ships, ask. Most stores would rather cancel than take a return.
Do it rarely. A buyer who cancels repeatedly is a buyer stores start blocking, and that is their right.
What you cannot do
Cancel an auction win because you regret the price. A bid is a promise to buy at that price. Is a bid legally binding?
Change the item. A different thing is a different order.
Undo a pickup choice after the collection window has started, without talking to the store — there may be a no-show fee attached. What does "collect by" mean?
If the store cancels instead
You are refunded in full including delivery, it costs you nothing, and it counts against them rather than you. The seller cancelled my order
The thing that prevents most of this
Check the address before you pay, not after. It is thirty seconds, and it is the single commonest reason people need this page.
And for anything where size or fit matters, ask for the measurement before you bid rather than the size. Five questions worth asking before you bid
More: can I change my address, how returns work, how to message a seller about an order.