If I'm going book shopping, or going anywhere that doesn't involve trolleys, I'd take my Carradice Bike Bureau along. I'll admit I don't really shop anywhere with narrow aisles. I think it depends on one's own shopping profile. For you, clearly the shoulder strap on the grocery pannier is essential, but I haven't found it necessary. I certainly didn't want to sound dogmatic about the necessity or otherwise of shoulder straps.

What I like about this forum is the multiplicity of solutions offered that one can fit to one's situation.

My problem is buying just that little bit too much to fit everything comfortably. I'd like some sort of open rack box thing, but one that wouldn't make it cover the rails for the panniers.