Trading Standards Notified
Unfortunately, this company breaches consumer rights based on their policies they express on the website.
The notice on the page (“cancellation fees apply, which is the cost of bank fees (between 4 and 10% depending on payment methods) plus £3 admin”) and similar wording on the site’s policies attempts to charge exactly the kind of fee the regulations prohibit for exercising the statutory right to cancel. Bank/processing fees and admin costs incurred by the seller on a refund are the seller’s problem to absorb, not something they can pass on to the consumer for a cooling-off cancellation.
For most distance/online sales of goods (like the Pokémon TCG Rival Battle Deck), consumers have a statutory 14-day cooling-off period to cancel for any reason, without needing to give a reason.
The period normally runs from the day after you receive the goods.
On valid cancellation, the trader must reimburse all payments received (including standard delivery) without undue delay, generally within 14 days of getting the goods back or proof of return.
Regulation 29 explicitly says the consumer can cancel “without incurring any liability except under these provisions” (the limited exceptions are things like the extra cost of non-standard delivery you chose, or a deduction for diminished value if you handled the goods beyond what was necessary to inspect them).
Regulation 34(8) is clear: “The trader must not impose any fee on the consumer in respect of the reimbursement.”