I had this issue throughout last year. One time I physically had to go to YYC from my home to book the ticket in person with my card because it was for a next day flight. I was livid. And I had the same situation with the call centre. I said you should waive the admin fee if your site is not allowing me to book. He wouldn't budge. I tried 2 US cards and 1 Canadian card online. Wouldn't process. Tried the same 3 with the unhelpful and rude call centre agent. He couldn't get the payments to go through.

This happened for 4-5 months last year and during that time I just booked with United and Air Canada. Was absolutely maddening.

Randomly it just stopped being an issue in December. I don't think it has to do with using a Canadian vs foreign card. I regularly use a non-Canadian card to book with them.