Much as I share your frustration, I think the manager is probably the wrong target for your venom, as he's just a wheel in a horrid corporate machine. I doubt it was his decision to install self-service checkouts or to cut staff. Although the latter is possible, I suppose, and if it isn't a factor then it is his responsibility to staff the shop adequately at any given point. A twenty minute wait in a checkout queue is clearly unacceptable.