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More of a legal requirement really. Provided the seller states in writing that they'll reimburse return postage costs (which they do in their Ts&Cs) they're not obliged to provide a pre-paid return label or collect the goods.
In my case, I prioritised getting my 90 quid back for the seat post over and above the postage. Luckily, their system, presumably automated, was fairly swift to refund for the item.
You paid to send back their mistake? There's your error