the term 'PARTY STORE' is a michigan-specific phrase for a shop that sells mainly liquor, beer, lotto, pizza, and other convenient drinks, foods, and items.

In fact, here is what Urban Dictionary says: Michigan slang term that describes a small convenience store that sells soda, beer, liquor, snacks such as potato chips and hot dogs, and some household goods such as plastic cups and paper plates. So there you go. A "party store" can be a "pony keg" and vice versa.