I don't know, I thought it was just taxes and fees, it was the max Avios allowed. First I called the number on the site, only on hold 16min which was great except they couldn't help with an Avios flight.

Called the number they gave me for EC bookings, on hold for 1:00:34 and call dropped (maybe my carrier has 1hr limit). Called again and got an agent, wait must have been about 50min because entire call was 1:05:06. I did reply "okay I'm holding" every few times the automated "please continue to hold" came on, hoping it might trick my carrier into thinking it was a real call.

Agent did the cancellation, points are back, and I have email confirmation of 100% refund to CC (7-10 days). Agent confirmed can't do right-after-booking refund online (only voucher). Also said due to long wait times, they've extended deadline from 24h to 48h. I don't think I'd count on it, but it's good to know to keep trying if tried and failed within 24 hours.