I’m not really offended by the asking for donations at the register thing, as long as those donations go 100% to the charity and it’s a good charity, then doing something to make it easy for people to contribute who wouldn’t otherwise take the time is ultimately a good thing.
The one that annoys me is where they match the donations. It feels like a method to guilt people into it by making their refusal to donate $1 into $2 not being donated.