I have successfully bid on an upgrade on an AC flight before. I had actually called AMEX to tell them that within the next 48 hrs a charge might be coming through, and gave the amount.... and the charge was declined by Amex because it came through under a name other than Air Canada. It may well be that they use a third party domain but of course proceed with caution! I wish I could remember what name it was as I have another bid in on a flight in September and would love to avoid this happening again!