Consumers running low on paper towels or dish soap used to run to the store. Now, the needed item could already be in the mail.

New sites Soap.com (a sister site to Diapers.com) and Alice.com aim to keep shoppers in supply of common household goods. Their pitch: fast and often free shipping, competitive coupons and regular reminders when (by their estimates) you re running low.

The regularity with which consumers buy household goods appeals to web retailers, says Dawn Iacobucci, a professor of marketing at the Owen Graduate School of Management at Vanderbilt University. Even Amazon.com offers groceries and home products. They re all hoping to hook you on the convenience of home delivery for a regular order, she says.

But that convenience does come with a cost. Although online prices are typically lower than those in store, avid coupon clippers can often pay less in person by matching store sales and coupons, says Teri Gault, the founder of shopping site The Grocery Game.

Here s what to consider before stocking up online:

Manufacturers coupons

Personal care and household items account for about half of available manufacturers coupons, says Stephanie Nelson, a.k.a. The Coupon Mom. Drug stores especially emphasize that, she says. Consumers can game the system by getting free products each week.

If you re set on buying online, the wealth of manufacturers' coupons gives Soap.com an edge -- it s the only site to accept them. The catch: You ll need to remember to mail them in about a week in advance for processing before you place an order.

Sales

Both sites and stores offer a weekly rotation of sales and exclusive coupons. Compare to see which has the best deal, Gault says.

Brand loyalty

If you re a fan of a specific brand, manufacturers have more leeway online to offer exclusive deals, coupons and free samples, Iacobucci says. On the other hand, you re less likely to see the cheapest generic versions, because there s no retailer to back up that store label.

Social media

Shopping online can be rewarding if you share the deals, Nelson says. Alice.com offers bonus discounts on select items in exchange for posting the deal on Facebook. A $3.49 Radius Kidz toothbrush, for example, drops to $1.49 after a $1 coupon and $1 sharing bonus -- a 57% discount. At Soap.com, each friend a member refers to the site gets a 15% discount on her first order, while the referring member gets a $10 credit.

Impulse spending

In stores, it s easier to make impulse buys that can ratchet up your bill, Nelson says. But online, it s just as easy to buy more than needed to reach the free shipping threshold.