I haven't found that shell stretches that much. What I've thought was shell stretching, was actually my foot settling into the footbed. I have heard that putting pressure on shell from the underside (w/ a shoe stretcher) is dangerous. Shell has a tendency to crack or split when you do this. Calf is much more forgiving and will expand.
I have an Amex business card that can override this. I haven't tried it in a long time, but they will back you up of you don't want an item, wrong size and, of course, defect. Generic MC and visa won't do this.