Pick n' mix is an institution here in the UK, although I gather it's on the decline.

It's not just a bag of random candy. You get a small bag, you choose from a specific selection of candy, and it typically costs an absolute bomb. You pay by weight, if I remember rightly. It's been a while for me, so I might be wrong.

They're traditionally very popular in cinemas here. When I was small, I always used to nag my parents to let me have some when we went to see a film.