Unless you agreed at the time of entering the contract to return rejected goods then your only obligation is to make them available for collection & take reasonable care of them.
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(7)From the time when the right is exercised—
(a)the trader has a duty to give the consumer a refund, subject to subsection (18), and
(b)the consumer has a duty to make the goods available for collection by the trader or (if there is an agreement for the consumer to return rejected goods) to return them as agreed.
(8)Whether or not the consumer has a duty to return the rejected goods, the trader must bear any reasonable costs of returning them, other than any costs incurred by the consumer in returning the goods in person to the place where the consumer took physical possession of them.
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Even if you have agreed to return rejected goods, they're in breach of contract and therefore are liable for all reasonable costs incurred. If you did agree to return, remind them of this and whether they feel confident that a court would agree return courier costs were not reasonable.
If you haven't agree to return rejected goods then remind them that it is their responsibility to collect under the Consumer Rights Act.
Money doesn't solve poverty.....it creates it.