ABC News' Becky Worley reports:

A new Web service called MySupermarket.com says it will deliver groceries to your home for less than you'd pay if you drove around town and shopped at a combination of grocery, pharmacy and big-box stores.

Like many Americans, I buy fresh food and dry goods at the grocery store, toiletries at the pharmacy chain and every few weeks head to a big box store like Costco or Target for cleaning supplies, household items and paper products. I do this to find the specific brands I like and get the best possible prices.

But the bargain and brand hunt is time consuming. So the promise of a Web-based service that would reduce the hassle and gas expense sounded good.

Mysupermarket.com compares prices of 55,000 items from eight leading online stores: Amazon, Walmart, Target, Drugstore.com, Diapers.com, Walgreens, Soap.com and Costco.com. You browse through name brand items on the Mysupermarket site and they look for the lowest price from all eight of those retailers.

I made a list of 12 items: batteries, paper towels, toilet paper, dog treats, trail mix, cereal, plastic bags, oatmeal, whole grain pancake mix, toothpaste, and a few name-brand cleaning products. The site's inventory lacks fresh produce, but I was impressed by the range of food and dry-good brands available. They had a favorite kind of trail mix I can normally only find at Whole Foods, Arm & Hammer toothpaste and all the cleaning brands I'm used to buying at Target or Walmart.

5 Tips to Save Big on Car Rentals

MySupermarket.com lets you compare items by unit price, listing options cheapest to most expensive. It's apples to apples across all the stores so you can truly choose the lowest priced option. They might suggest switching to a different size or quantity of the item to get a better price, and they'll search for coupon codes and free-shipping offers to sweeten the deal.

Most of the sites they index have free shipping on orders exceeding $25 or $50 dollars, but as they explain on their site, "If you checkout with a cart above $75 and we don't find a way to eliminate all your shipping costs, we'll reimburse any shipping fees remaining after shipping optimization."

In my order, I ended up with $2 in shipping costs, which MySupermarket refunded to me directly via Paypal.

You actually purchase directly from the stores MySupermaket lists, you don't purchase or do returns directly with them. When it comes time to check out, you must go through a one-time signup with any of the eight online stores from which you purchase (or input your existing account info), but MySupermarket.com automates the ordering process on subsequent purchases so it's pretty seamless.

Total cost for my 12 items online (many of which were bulk purchases) was $131.52 with $7.93 in sales tax for a total of $139.45. Total shopping time 20 minutes, but delivery of the items will take between two and seven days, depending on the outlet.

To compare the new online experience to my normal shopping methods, I set out on my usual quest: Costco for the batteries, paper items and any of my favorite brands I can find, the supermarket for cereal and most food items, the pharmacy for cheaper prices on toiletries and, finally, to Whole Foods for specialty items I can't find at my other stores.

I do find better unit prices at Costco on many of the big bulk paper products, the brand-name batteries and the cleaning supplies. On average the prices were about 20 percent cheaper than the comparable prices on MySupermarket.

While Costco.com is one of the companies MySupermarket indexes, the .com side of Costco has significantly limited inventory compared to their physical stores. And the reason for that seems obvious: Costco can't keep the margins as low online with shipping costs and they depend on your impulse buys from the center of their stores to augment loss leaders and low-margin products.

Tips to Save You $1,000 on Your Energy Bill

While I'd like to complete my purchases at Costco, the brand selection is limited and some of the products I am stuck on (I have a relationship with my toothpaste, sorry) weren't available. So I head out to three other stores to get the items I need at the best possible prices.

At all the other stores, the items I want are more expensive than those I found at MySupermarket.com. Three hours and 31 miles later, I have spent a total of $141.62, including tax, but not including the cost of my gas or time. On the upside, I have my items immediately and don't have to wait for delivery.

The My Supermarket items all arrived without incident in the promised amount of time, two to seven days, and I will use the service again. It was super-convenient, but sometimes you just need the toilet paper, pancake mix or batteries right this instant, so the online option will be a tool I use when I'm organized and proactive, not scrambling, as I normally am.