Zipolite has a very meager selection of groceries, fruits / veggies and booze. Basically 2 small stores for the canned and dry gods and one veggie and fruit place that often has wilted and overripe stuff ! I purchase my beer from the Corona "deposito " at the far east end of the main paved street...by the veggie place. When you first purchase you pay for the bottle and liquid and then return your empties and pay for only the liquid. Before you leave, return the bottles for cash. I think they were closed on Mondays. There is also a very small Italian cafe with a deli section with some salami and good cheese, but prices are double what I pay at home.
If you have a kitchen where you rent and will be cooking , make a trip into Puerto Angel ( camioneta or taxi ) and go to the best grocery store in town called Siete Regiones. Its an orange colored building on the main street at the west end of town close to the street that goes down to Playa Panteon. It has a good selection of hams, cheeses, juices, canned goods and wines and other booze ...cheaper than Zipo and way cheaper than San Agustinillo. ( great beach in San Agustinillo ) .Not far from this store a couple of blocks east over the bridge to your left is a small market with a good selection of fruits, veggies and bread. We have stayed in Puerto Angel and gone down to the beach by the pier to purchase fresh fish as its being brought in, in the mornings.
If you need a better selection of things...like say a hardware store, go into Pochutla. Monday I believe is their market day. There is a large Mexican version of Wallmart as you go into town on the right hand side. I have purchased frozen meat there as I dont like the market chicken that is not refrigerated. You can catch a camioneta across the street to go back to Zipo.
People also go into Pochutla to banks and ATM machines. I got ripped off by the ATM in Zipo at the Nude hotel. The other one at the Playa Zipolite hotel never seemed to be working .... oooh... cheapest beers on the beach were at the Hotel Zipolite...15 pesos for a Corona
Water you can buy anywhere ( dont think you will find mineral water ) but many hotels and guest houses supply it. Make sure you buy the Mycrodine drops for the water to soak all veggies and fruit.
I always bring my sunblock, bug spray, creams , shampoo etc from home as you will pay a lot more there and you may not find what you need.
I dont think you will get anything more or better by going to Huatulco...probably the prices are higher and the trip is about one hour.