ESH, Everybody Sucks Here.

The employee starting with the phrase "Just so you know..." is going to be hitting a hot button for a lot of people. If any employee would be offering that information, it would probably be better coming from the cashier: the line guy saying it makes it seem like "Your big order is a pain, go online and order."

The guy yelling was obviously out of line, so there's that.

You could certainly have said something. But you fired a warning shot with "Are you kidding me" instead of just going "Whoa, dude, these people don't deserve that" or something similar, not "Stop it!" like you're a kindergarten teacher. And you admit "here’s where I get louder!" so you were a yelling asshole too.