He was way out of line, not you imo. I have no problem tipping 20-25% for a wait staff who's actually providing me a "service", but handing me something and taking my $ at the register doesn't qualify imo.
ps: I do occasionally order take-out from a local Mexican place...where the person (typically a wait staff) will take my order, package it up, etc. They get a tip
pps: A $14 breakfast burrito??? For that, I think I'd have driven down to San Diego...Santanas (we always called it...)...."Purple Burrito Heaven"....filling and cheap...still give a buddy crap about eating TWO of their humongous 1 lb'ers one time