Thank you for explaining it, your grammar certainly puts mine to shame.
I understand "Penut butter and jelly" is the name of a sandwich, so it automatically counts as a unit which makes it a no-brainer for me .
In other hand, Champagne and popcorn are so far apart. My first reaction was the same as TK and it took me some thinking to come to turns with it.
Silly example, but it works for me....
A: What's for dinner, bro? B: Cheese burger and coke is my dinner for the night. (I haven't heard people say Cheese burger and coke are my dinner for the night.) A: Seriously??? That sucks!!! B: Naaaa, Cheese burger and coke is better than you think. (Cheese burger and cake is my dinner combination and I count them as one unit.)