nelly, that’s a shame. My experience is that Hush doesn’t honour offer promo prices on exchanges after the promo has passed - I sent a few things back a couple of months ago asking for an exchange, but they just refunded me, even though the items were in stock. Your best bet is probably a personal appeal to them!
Re sizing, I’m wearing an S in the Beatrice at a size 12, and I could probably go to an XS except the arms might feel a bit tight.