Well, presumably this would be configurable in the mobile app / UI so users can choose how to (order etc) liquidate their holdings in order to fund the card.

Obviously there could be some sane defaults. By default aggregate all card spending against user's margin account, every day/week/month sell the cheapest stock which covers outstanding balance (so if you spent $150, it would first sell 1 share of Apple, it wouldn't start selling Amazon until you run out of Apple shares).