Gotogate Refuses to Refund My Voucher After the Airline Cancelled My Flight

I reside in Istanbul, and I purchased my plane ticket through Skyscanner via Gotogate. My flight with Kuwait Airways, scheduled for June 18th, was cancelled entirely without my involvement by the airline itself, and the transportation service I purchased was never provided at all.
For this booking, I used a voucher worth $894, and I also made additional payments from my credit card, including a $412 fare difference fee, a $137 seat reservation fee, a $23 special diet fee, and a $17 check-in fee. My reference numbers for this Gotogate booking are 84M3F9 and 1126-970-344.
After the cancellation, I communicated with Gotogate both in writing and by phone. First they created a new booking, then cancelled that one as well. Afterward, they said they could issue a refund, and I agreed to that. However, when I asked about the total amount I would receive, they told me that only the $412 fare difference fee, the $137 seat reservation fee, and the $23 special diet fee would be refunded, while the $894 voucher amount would not be refunded at all on the grounds that it was single-use. For this reason, I have not yet confirmed the cancellation.
Given that the flight was cancelled by the airline and the service was never provided at all, having such a large voucher amount completely forfeited — with no refund and no new voucher issued in its place — goes against both fairness and reasonable consumer expectations. I am requesting that all amounts I paid be refunded to me without any loss: the payments made by credit card refunded to my credit card, and the $894 voucher amount either reissued as a new voucher or otherwise made usable for me in an appropriate way.