In British English, a Haberdasher sold cloth, as well as lots of little 'odds and ends' - the 'notions' named for those items which you might forget or not be able to get elsewhere. This often included miscellaneous goods - needles, thread, buttons, glue, blotting paper etc, and haberdasheries entered a grey area as something between a tailor and a hardware store.
In the remote location where the movie was set, the Minnie's is going to be your only chance to buy any oddments you might have forgotten for your journey - something like a general store or a highway service station shop - no one is doing their weekly grocery shop there, but might pick up some odds and ends in passing.
It was probably referred to as a Haberdashery as somewhere to pick up miscellaneous items - but not a grocery or wholesaler. Given it's isolation, and Minnie's nose for business it's inevitable it would provide other items for passing trade - hot food, coffee, booze, candy etc like a service station.