If you let a dispatch policy filter, it can push a courier past their capacity. So ours doesn't get to: the constraints sit in one fixed filter, and the plug-in gets nothing but a sort key over whatever survived. Worst case it orders badly.
The key ends with the worker name, so exact ties get decided by the candidates, and the same roster gives you the same assignment every time. https://shipmindlabs.com/c/6ddb2ace