Supplier refusing to reimburse after booking is cancelled.

Stellenbosch Suit Hire refuses to accept a cancellation of the hiring of a garment. I paid the full amount of the hiringon the 25th of Oct 2018 to book the garment for the 10th of November 2018. I cancelled the booking on the 9th of November 2018, and while I don't expect to be fully reimbursed for cancelling the hiring, the CPA protects me by saying that the supplier may charge a reasonable cancellation fee. Stellenbosch Suit Hire refuses to even agree to a reasonable cancellation fee and insists that they are well within their rights to refuse to reimburse me, because they've stated on their booking slips that they "don't accept cancellations". They accepted my full payment, so why can they not meet me halfway by agreeing to reimburse me the difference between their deposit and the full hiring amount?