Sarah--It's kind of cheating but my philosophy is usually it's better to ask forgiveness than permission.
You should have phrased your e-mail differently, just being so excited about the candy bar and how some are really expensive, but you did some great bargain shopping, etc...You aren't ever supposed to start a letter,conversation, etc. in a negative fashion. Don't acknowledge that she could not like it.
Bottom line, it's your wedding, regardless of where it is being held, so do what you want! I've seen lots of candy bars, and none of them were ever tacky! When is your AHR? A coffee bar in the summer will not be used, even by old people. Trust me! I'm an event planner.
Oooh! You should tell her you consulted with a planner who said they were the hottest tends in weddings and that coffee bars are passe!