The Rakuten shopping portal gives you cash back for purchases you were already planning to make

When you think of ways to earn cash back for your purchases, popular credit card offers might quickly come to mind. But shopping portals can be an equally effective way to capitalize on cash back offers for items you have to purchase anyway — and with a shopping portal like Rakuten, you don't have to worry about paying any fees or any interest charges for cash back and other perks.

Rakuten is a pretty low-lift way to score cash back for online and in-store purchases. The cash back you earn can be used for literally anything, too — it can be extra spending money, you can use it to boost your savings and investments or it can be used to make extra payments toward high-interest debt. It may not always seem like you're getting back a ton of money, but it can add up over time.

CNBC Select analyzed the Rakuten app for factors like perks, fees and referral policies. Here's what you need to know if you're thinking about signing up.

• How to get started
• Cash back earnings
• Perks
• Fees
• Referrals
• Bottom line

To sign up for Rakuten, all you need to do is create an account with an email address and a password on their website. Once you do that, you can download the Rakuten browser extension — this will come in handy for quick activation when you're about to checkout online. Keep in mind that the brand must be part of Rakuten's network for you to receive cash back, and you must activate the browser extension every time you place an online order to receive the cash back. Alternatively, you can search for your preferred store on Rakuten's website and then click through to the retailers website, this activates tracking to ensure you get cash back once you make an eligible purchase.

You can easily double check to see if the brand is part of the network by typing it into the Rakuten homepage, or keeping an eye out for the Rakuten cash back banner that will automatically pop up on the brand's page to remind you that you can receive cash back from the brand.

You can also download the Rakuten app for your phone and add your credit cards and debit cards to your Rakuten wallet so any in-store purchases you make with those cards will be eligible for cash back offers.

When it comes to actually receiving your cash back earnings, you'll need to provide either PayPal information so they can digitally send your cash back or a mailing address so they can mail you a check.

You can browse various cash back offers on the Rakuten homepage. For the most part, the amount of cash back you receive will depend on the brand you're ordering from; cash back can be as low as 1% and as much as around 20% — and there's no limit to how much money you can cash out.

For example, when clicking through Rakuten you might typically earn 3% cash back from purchases made at Ulta Beauty, 2.5% cash back from GameStop, 3% from JCPenney and 1.5% from Nike. So let's say you spend $100 at Ulta Beauty; you'll earn $3 for that purchase.

More popular retailers, like Walmart and Amazon, have more restrictions on what you can earn cash back for. You can currently receive 1% cash back from purchasing certain items from Walmart — Rakuten cash back is not available for purchases on furniture, health, camping gear and Apple products, to name a few. Purchases from Amazon are not currently eligible to receive cash back.

You'll receive your total cash back earnings on a quarterly basis through either PayPal or a physical check mailed to your address (depending on which option you chose). You can go into your account to view your cash back earnings history at any time, and it will also tell you on what date you can expect to receive your next payment.

If for some reason you notice that you are missing cash back from a recent shopping trip or online order, it's easy to submit a support ticket to make sure you receive those earnings. You'll need to log in to Rakuten, navigate to the Help page, click on the "Missing Cash Back" option and fill out the details of your order, including the date you placed the order and the amount you paid.

And if you're an American Express cardholder with a card that earns Amex Membership Rewards® points, you can choose to earn Amex rewards points instead of cash back for purchases you make through Rakuten. In this case, the cash back you earn from Rakuten will be converted to rewards points and transferred to your Amex account. Just keep in mind that if you choose to do this, your Rakuten account will only be able to earn rewards points and not cash back.

Amex cards that earn Membership Rewards points include the American Express® Gold Card and The Platinum Card® from American Express.

Rakuten offers a $10 welcome bonus for new sign-ups.

Occasionally, some of the stores and brands on Rakuten's network will have what's called double cash back offers. So let's say H&M usually offers 2% cash back on orders through Rakuten — with double cash back, you can now receive 4% back on your order. It's a great way for you to earn even more for items you have to order anyway.

Similar to the PayPal Honey browser extension, Rakuten also offers some discount codes for select shops (though, Rakuten's discount codes are not as expansive as Honey's). It's a nice way to pay a little less and receive cash back at the same time, especially since you cannot have Rakuten activated while also using another browser extension to save on your orders.

And to help put even more money back in your wallet, Rakuten even offers a credit card: the Rakuten Cash Back Visa® Credit Card. The card lets you earn an extra 3% when you shop through Rakuten (on top of the cash back you'd normally earn for having the browser extension activated). And for purchases from brands that aren't part of Rakuten's network, you can still earn 1% cash back when you pay with the card. And to top it all off, the card doesn't carry an annual fee.

For a credit card option with a slightly higher cash-back potential, though, you might consider the Citi Double Cash® Card since it earns 2% cash back on all purchases; 1% on purchases and 1% after you pay your bill. It's most beneficial to pay for purchases using a rewards credit card while simultaneously clicking through a shopping portal. This way, you can earn double the rewards: cash back from the credit card and cash back from the shopping portal.

Rakuten is completely free to use and there are no fees or hidden costs.

Another great perk is the ability to earn extra cash every time you refer a friend and they sign up for Rakuten using your link. You can currently earn $30 per referral.

Rakuten is a great way to help you earn some cash back for purchases you need or want to make anyway. It's very simple to get set up and earn cash back as long as the brand you're purchasing from is part of Rakuten's network.

Ordering from a brand that's offering double cash back for a limited time is also a great way to boost your earnings. And the quarterly PayPal deposits (or mailed checks) give you something to look forward to.

For rates and fees of the Platinum Card® from Amex, click here.

For rates and fees of the American Express® Gold Card, click here.