We compared grocery prices at Costco and Boxed— here's which store will save you more money

Most shoppers know that buying in bulk can net you some serious savings, which is one of the many reasons Costco is one of America's favorite places to shop.

But is buying in bulk just as effective when you order online? Starting in 2013, online wholesale retailer Boxed.com began delivering groceries and household staples to the continental U.S. with two-day shipping. Costco launched its home-delivery option in 2017.

To determine which store generally has the lowest prices for bulk items ordered online, CNBC Make It compared two-day shipping prices listed online from Boxed and Costco on a range of everyday items for delivery in New York City. When items were not available through two-day shipping, we compared prices for Boxed's Express Grocery, which handles perishable foods, to Costco's same-day delivery costs. We evaluated both national brands and each company's store brand when those were available.

After crunching the numbers on 26 grocery items — ranging from fresh produce to baking ingredients to meat to household items such as trash bags and laundry detergent — Costco came out ahead in 16 categories where both retailers sold a similar product.

Here's how each retailer stacked up:

When the Krazy Coupon Lady, a website that provides the latest coupons and retailer deals, compared the two retailers on over 50 national branded products, it also found that Costco prices beat Boxed, co-founder Joanie Demer tells CNBC Make It. In their analysis, the Costco prices on groceries and household items were, on average, 17% lower than those offered by Boxed.

While we verified the prices listed above with Boxed, Costco declined to verify its prices since, a spokeswoman said, they can change on a daily basis.

While Costco generally offers affordable options across the board, there were three items where the retailer provided substantial savings over Boxed's prices: olive oil, flour and bananas. In fact, experts say Costco's olive oil is a top value.

Independent consumer testing organization ConsumerLabs analyzed 10 popular olive oils to see which provides the best quality and value. It named Kirkland's $16.99 organic extra-virgin olive oil one of the top three, beating out more expensive brands including Colavita and Newman's Own.

In our comparison, we found that Boxed was most competitive on many household items, such as laundry detergent and toilet paper, as well as pantry items, such as honey and mixed nuts sold under its Prince & Spring brand.

But the products offered through its Express service, which generally includes perishable items, are not as cheaply priced.

A company spokesman says that, in offering those products, Boxed is more focused on customer convenience. "For us, fresh produce is less about offering the lowest price on the market and evolved from a need we saw customers in our key markets have for a new, convenient way to grocery shop online from a curated, on-demand selection," he says.

The Express service is currently available in Atlanta, Boston, Dallas and New York City, as well as in many areas in New Jersey.

Demer says that if you do find Boxed items that you like, shop the online retailer's sale section. "Twenty percent is the minimum savings I'm looking for whenever I shop Boxed," she says.

Plus, the site often offers coupon codes, so check out RetailMeNot for the latest deals, Demer says. American Express cardholders can also earn up to 10 membership rewards points for every dollar spent at Boxed.

Boxed does not charge customers a membership fee, although you can join the $49 annual BoxedUp program to earn extra perks and rewards.

At Costco, though, you typically need your own paid annual membership, which costs $60 a year for the basic version. There are ways to avoid paying, but if you're a regular shopper it makes sense to shell out for one. You only need to spend $350 per year at Costco to save enough to break even on the cost of your membership, Demer says.

"That membership fee allows them to reduce markups lower than any other stores, making Costco and BJ's and Sam's Club prices often 10% lower than you'll see anywhere else," Demer adds.

For our comparison, we evaluated Costco's prices that are available to members through two-day shipping to make it as comparable to Boxed as possible. However, the retailer says that items may be cheaper when shopping in-store. Consumer Reports found that grocery prices for same-day delivery were 31% higher than the ones in the store.

To qualify for free shipping on your two-day delivery from Costco, you need to spend a minimum of $75. For same-day delivery through Instacart, prices include a service and delivery fee. With Instacart, local shoppers will buy the items you request from Costco, as well as other participating stores, and deliver to your doorstep. The minimum order for that service is $35, according to Costco.

Boxed, on the other hand, offers free two-day shipping on orders of $49 or more. For orders less than $49, delivery costs $6.99. The company also provides free shipping on your first order if you spend at least $20.

Also, with home delivery, consumers do not need to spend money on gas or associated travel costs of getting to the store.

No matter where you shop, make sure you're being smart when it comes to the larger quantities sold by Boxed and Costco. You need to be able to eat any perishable food before it goes bad, for example. If you live by yourself, springing for Costco's 1-pound container of fresh spinach may not make sense, even though it is a steal at $5.49.

"The thing you have to do is to remember to shop strategically and, wherever possible, stick to a list and a budget," Demer says.

Don't miss:

• 9 foods and grocery items you should always buy at Costco
• 4 ways to shop at Costco without a membership that could save you money

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