For last minute shopping on the 24th, I think the only options are likely to be the shops that are usually open on Sundays: i.e. the shops in the arcade attached to the Louvre (which include more than just your typical museum gift shop), and about half the shops in the Marais neighbourhood, where the department store BHV is located. (That might explain why BHV has decided to allocate one of its scarce Sunday opening rights to Dec 24 - since other shops in the area will pull in customers, too.) I haven't been to the Champs Elysees on a Sunday but I understand that some of the shops there also tend to open on Sundays.