I think what everyone seems to be missing is the letter was written to Mrs Claus - she was granting a request made to her, not doing her husband's bidding. Not quite sure why the little boy was writing to her not her husband though - I'm not sure a six year old boy would have any concept that Mrs Claus might be better at shoes, being a woman and all 
Tbh what I find much more classist than a MC family on an M&S ad (you don't say!) is this stereotypical notion here about WC families and trampolines. Yuck!
And how do we know that 'Mrs C stays at home to do all the hard work' anyway? We don't know what she does every other day of the year. Making some pretty sexist assumptions there! 
The ad is clearly aimed at a middle-aged, middle-class, female demographic - and you know what, it's not sexist to suggest that that's the demographic which will be doing the most spending over Christmas. Look at this very Christmas board - legions of us, getting overexcited and massively enthusiastic about everything from stocking fillers to table decorations. The forums my DH uses don't have men discussing the best way to stuff a turkey or what to buy the MiL who has everything, much less a whole board dedicated to Christmas. Perhaps if we told them to do it all themselves, the forums would have stuff like that on them. (And before I'm flamed, I know there's loads of men who do all the cooking at Christmas, and all year round.)
If we love all the preparation, that's fine, and it's not sexist. It's only sexist if you don't have a choice in the matter, and it's just expected of you, and tasks aren't shared equally. And I can't see that happening to Mrs C in this ad at all. She pilots a helicopter ffs, how cool is that?!
Oh and Amy, you're pretty great. Never remove those twistable knickers 