Last week I flew from YYZ to LGA as part of a three leg trip on a ticket purchased through my corporate travel agency on UA ticket stock 016.

The day of YYZ-LGA travel (snowy day with lots of flight cancellations) my flight was canceled and I was confirmed on a later flight.

While in ML lounge waiting I went online to re-enter my UA FF number which somehow got lost in the re-booking. When I was in my record I noticed it offered an option for an upgrade to F. I thought what the heck - though I was surprised as several canceled flights - I would have thought F would be full of pax from those flights.

When I tried to complete the upgrade request it refused to process it saying this offer is no longer available. Okay.

The flight was packed, as expected, there were some seat reassignments due to their getting a larger plane.

About 4 days later a charge showed up on my CC for the "upgrade" (I assume it was for the non-upgrade, it said "Air Canada 016..... - my ticket number) and the amount roughly what I remember for the offer.

What's the best way of contesting this charge? Should I try to contact AC or just let me CC deal with it? I've got the BPs I used, so I can prove I traveled in Y is there a chance they'll try to argue I was actually in F and I can't prove otherwise?