Messenger / Courier Bags List

I can't see a cross strap on that, so I suspect it'd be irritating on the bike. The better bags (anything you'd want to consider buying, basically) have a second strap that doesn't take any load, but stops the bag sliding about on your back.

Good point. I just checked the description & it says:
*• Cross-stabilizing strap around waist
*Its probably taken off for the photos.