Best way not to get ripped off is have your dealer weighh your shit out for you.. I mean if you're talkin about ounces or even halves then it should always be weighed without question. Different strains and qualities can look veryyyy different. I mean I've seen nice fluffy ounces of some real shit that looked like it was a pound and then I've seen 30.0g nugs of schwagg smaller than my fist. My point is that density cannot be determined with your eyes and if you're talkin about lower quality bud then seeds and stems also play a role in it.. and yeah a "zip" is just slang for an ounce. My guess is it was derived from "zip-lock" bags that are usually the best for holding this quantity or some shit...

Stay high