I've used WeGoShop.com several times.
You can pick the grocery store. (We like Publix.) You can even check the ad online.
You submit a free-form list to Marge by e-mail ahead of time. You can be specific, specifying the exact brand, description, and size (such as "Yoplait Original Yogurt, Mixed Berry, 6 oz."). Or you can be general (such as "small berry yogurt"). Or you can mix the two.
You pay actual grocery store prices, plus a reasonable service charge (and a tip if you're happy with the service).