Adam Garrett
Rakuten: My Favorite Automatic Chrome Option to Stack Points

Image courtesy Rakuten on my account
I have a few favorite Chrome installations for savings online (shopping as well as other things like online services) that tell me how many points per dollar I can get while shopping on a website:
Rakuten (referral link where we can each get $30 after you spend $30)
While I suggest having all downloaded, my favorite of the 3 by far is Rakuten in terms of being the typically best bet.
I recommend:
A. Getting all 3 including Chrome installations so that you're notified of what you'll get without having to spend time going on a separate website.
B. Get the Rakuten Cashback to convert to Amex Membership Rewards at a 1:1 ratio (which doubles the value according to TPG valuations)
C. If you have viable options with Alaskan Air Miles (even when flying other carriers), I generally recommend earning Alaskan Miles over Rakuten when Alaskan Air is offering more than Rakuten. It could be argued that when Alaskan is offering the same amount that you should go for the Alaskan miles because Alaskan Miles are more difficult to earn than Amex MR. In most cases, Alaskan Miles in my experience offers less than Rakuten, but watch out for when that's not the case.
What credit cards to use on Amazon
The Amazon Prime (personal version & business versions) are great options to get for initial spending if you spend a lot on Amazon and have prime.
Other credit cards for online shopping that isn't a particular niche
My favorite card for spending on other shopping online is the Bank of America Customized Cash Rewards card. Because I'm a Preferred Rewards gold member, I get a 25% bonus on the 3% category I've picked of online shopping for up to $2500/quarter, giving me effectively 3.75% cashback. The way that I am Bank of America Preferred Rewards Gold is due to me having between $20k and <$50k in my Bank of America accounts. Since BoA doesn't offer much interest compared to my other accounts, the way that I have >$20k in my account is via a liquid stock account and Roth IRA account with stock. Fees are paid to the government for trades that are very low, while BoA charges me nothing. It's a win-win.
That said, if you don't have Preferred Rewards Gold or higher, or if you can get better value out of points cards, I'd probably recommend a 2x miles card of higher than 1.5 cents per point according to TPG valuations rather than 3% cashback with no Preferred Rewards status. Examples include:
1. Amex Blue Business Plus for $50k/yr in spending with no annual fee (I have 2 of these) [I sometimes have referral links for these]
2. Citi Double Cash if you are willing to convert that cashback into points via the Citi Premier or Citi Prestige, especially if you also have the Citi Rewards+ which gets a 10% bonus on converted points up to 100k points converted to partners per year.
3. Any 2x Capital One Miles cards or cashback equivalents if you have a miles card that you can convert the miles to. [I sometimes have referral links for these]
Other credit cards for online spending in niches
The Amex Platinum is a solid card for 5x at airlines when spent directly at the airlines. It's also good for Saks Fifth Avenue purchases due to the $50 credits it gets every 1st half and second half of the year. It's also good for the credits on Uber eats. [I often have referral links for these]
Spending at office supplies, cable, internet, & phone is best often with the Chase Ink Cash card for 5% cashback up to $25k/yr that converts to Chase Ultimate Rewards when you have an annual fee incurring Chase UR card and make the transfer to Chase UR.
Spending on ads, as long as the merchant category code works for you, is often best on the Amex Business Gold if you do a lot of ad spend for 4x Amex MR, or if you do substantial but less ad spend, the Chase Ink Preferred for 3x Chase UR per dollar with a lower annual fee. [I often have referral links for these]
Spending on groceries is often best with the Amex gold if you get positive expected value vs alternatives, but in order to do that, it's typically best only if spending a lot on groceries at grocery stores AND using the $10 restaurant delivery credit every month due to the high annual fee of the Amex gold. If you wouldn't use that dining credit, you're likely better off with the Citi Premier. [I sometimes have referral links for the Amex gold, but never the Premier]
Here are some solid options on food delivery.
There are a plethora of other categories to consider, but the above are some of the top ones for many.
Referral Links
While in some cases I don't have referral links, when I sometimes do, I share it above. I'd sincerely appreciate you reaching out if you are about to apply in cases where I have a referral link in some cases. The referral links are regularly changed, so if I added a referral link now, they likely wouldn't last long.