Want some money for nothing? Many online retailers pay referral commissions to businesses that send them customers. Online shopping portals such as BeFrugal.com, CouponCabin.com, CouponFollow.com, MrRebates.com, Rakuten.com, and TopCashback.com give their customers a cut of these commissions and pocket the rest.

Our staff has become big fans of these sites and use them regularly. Rakuten just sent me nearly $90 for this quarter for using it to book a resort stay, file my taxes online, purchase a new fridge, buy clothes, and more.

The trick is to shop via the portals’ websites, or to download their browser extensions to make the process seamless, skipping the step of going to the portal’s site first. If you don’t have the extensions downloaded, to claim rebates from shopping at Banana Republic, you have to visit, say, Rakuten.com and begin your spree by accessing Banana Republic’s site through it. Otherwise, a retailer won’t know it owes Rakuten (and you) a commission.

Depending on the portal, affiliated retailer, type of purchase, and promotional bonuses, cashback returns range anywhere from one to 40 percent of what you spend. Some sites offer a registration reward when you sign up, and others have referral programs (at the time of this writing, Rakuten was offering $30 to both you and the person you referred). During the holidays and other special seasons, some portal sites offer incentives for spending a certain amount of money through them.

Your kickback depends on which cash-back portal you use and where you shop. Before making a big purchase, check first with CashbackMonitor.com, which aggregates offers for various rebate sites and reports the best current payouts. For example, we recently found that BeFrugal.com was offering 12 percent cash-back at Adidas (CouponFollow.com was offering 4.5 percent). For Hotels.com purchases, BeFrugal was offering seven percent cash-back (Rakuten was offering just one percent). For Banana Republic, TopCashback.com gave eight percent cashback (Mr. Rebate$ was offering three percent). For a Hello Fresh meal delivery subscription, USAA’s shopping portal was offering a flat $35 cash-back (Rakuten was offering $10).

Because the various rebate sites often offer different payback yields for the same retailer, it can pay to check several rebate sites for the best deal for each purchase. One way to save some time is to use another online tool that aggregates rebate offers and reports which ones have the best payouts for various retailers. We tried CashbackMonitor.com and found it bookmark-worthy.

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The cashback services generally also share retailers’ promo and coupon codes with their customers, and, unlike most discount programs, you can double-dip. You could, for example, buy something on sale, use a coupon code to save 30 percent off, pay with a credit card that provides its own cashback or points rebate, and still get a percent back from a rebate portal site. In some cases, the portals offer the only way to get any savings from certain retailers (for example, Apple was recently offering two percent back via Rakuten on certified refurbished products). StubHub was offering four percent cashback, also via Rakuten, and we all know it’s tough to score a discount on event tickets, especially for a popular Broadway show or can’t-miss concert.

We love these portals for travel and the payoff can be big. An eight percent cashback return for a stay at IHG hotels (as we recently saw on Mr. Rebate$) can add up quickly, especially for a multi-night stay. And remember, you can stack these offers on top of other deals or membership discounts (like AAA or AARP). Mr. Rebate$ was offering eight percent cash back on a rental from Hertz when we checked. RetailMeNot featured three percent cash back on a VRBO stay, maybe enough to take the family out for ice cream while you’re away.

Once your account has hit a certain balance or when the earning period closes (often each quarter) you have the option of getting the money deposited into your bank account, via PayPal, a snail-mailed check, etc.; each site has slightly different terms for timing and payment methods. Keep an eye on your account and make sure you are credited for your purchase. It can take several days (longer for hotel bookings, you typically don’t get paid until after the stay) for cash to appear in your online balance, but we sometimes need to follow up via online form or chat to get our money. Usually, it’s as easy as providing the total and order number and the balance is updated quickly.

Not all retailers play ball with the rebaters. You won’t find REI or Chewy on them for example, and some retailers such as Amazon, Target, and Walmart only pay off when you buy from certain departments. But it’s often worth checking for rebates, even when browsing on websites we didn’t think would participate. Sometimes the payout is very low, but getting even one percent back from buying essentials from Home Depot or an annual supply of contact lenses adds up, especially if you already purchase most of your stuff online.

A warning: when shopping using rebate portals, make sure you can’t find a better deal elsewhere. After all, it’s not worth going out of your way to use a portal to get back two percent on a purchase if you can buy the item or service elsewhere for 30 percent less. And avoid an overspending trap: Keep in mind that from a retailer’s perspective, the whole point of these commissions and rebate arrangements is to get consumers to spend more.

You’re also sacrificing some privacy to save some cash. The shopping portals use cookies to track what you buy and what they owe you. As with most online transactions, cashback sites will share or sell info about your searches and purchases with others. If you prefer to buy online privately, these services may not be for you, or you might be selective about which purchases you make via the cashback services. For what it’s worth, our shoppers didn’t notice an uptick in spam, telemarketing, or other annoyances after using the cashback services.

Avoid websites that ask you to pay membership fees; it’s a red flag—none of the major players charge them.