How can I be sure the cashback will actually come through?

You can't be certain. What you can do is make failure rare and recoverable: click through clean, buy in that same tab, skip outside coupon codes, and screenshot the confirmation before you close it. Shopping portals (sites that pay you cashback, miles, or points for starting your purchase there) are usually slow rather than broken, and BeFrugal and TopCashback both ask you to wait at least 7 days before filing a missing-cashback claim.

Someone on r/CreditCards put the question better than we can: is it "just best practice to assume you won't get anything back and be happy if you actually do?" That's a reasonable position, but you can stack the odds a bit. We haven't found a portal that publishes a tracking failure rate, and when tracking does fail, the difference between an annoying loss and a two-minute fix is whether you kept anything you can show a portal.

How to make sure cashback tracks, before you buy

Run this in order. It costs less time than reading about it costs, and it removes the common ways a purchase quietly fails to track.

1. Start at the portal, not the store. Search the store on the portal, click its link, and let it hand you off. A purchase that starts at the store, or at a search result, has no click-through behind it and nothing to credit.
2. Turn off the things fighting the click. Ad blockers and tracking blockers are one of the most common causes of a failed click. Pause yours for the portal and the store, or use a private browsing window (a few blockers keep running there too, so pausing is the surer move).
3. Finish in that same tab, in one sitting. No bookmark, no fresh search, no second click from a different site. Whichever link touched the store last is usually the one that gets paid, so a stray tab can quietly hand your cashback to someone else. That's last-click attribution, and it's the mechanic behind most "but I definitely clicked through" stories. The other avoidable failures are covered in the three tracking traps.
4. Use only the codes the portal itself shows you. An outside coupon code can reassign the commission to whoever published it.
5. Screenshot the confirmation page before you close it. Order number, total, date, all in one image. This is the step people skip and the step that matters when something goes wrong.
6. Note roughly when you clicked. Most portals log this themselves, and it's worth knowing where yours shows it. If you're signed in here, we save a click receipt for you: store, portal, the rate we showed, the terms, the time.

Two more things worth knowing before the purchase rather than after: whether a portal commonly excludes categories at that store, and how long that portal usually takes (every portal we track has a page carrying its recent moves). Rakuten's own help center puts the typical wait for a purchase to confirm at 3 to 14 weeks; BeFrugal's help page gives 15 to 60 days in one place and 30 to 90 days in another (the timeline, portal by portal). If you buy on Monday and check on Thursday, nothing has gone wrong yet. The cashback tracking guide covers every common cause of a miss in more depth.

Do coupon codes void cashback?

Sometimes, and it depends where the code came from. A code the portal itself shows you is the lower-risk kind, since the portal set it up. A code from a coupon site or a browser extension can overwrite the portal's cookie at checkout and take the commission with it, which is exactly what a portal means when its terms say cashback may be voided by codes from third parties. If you want the code more than the cashback, that's a fine trade to make deliberately. Making it by accident is the part worth avoiding.

How long should I wait before assuming it failed?

Wait at least 7 days: that's when a purchase should show up as pending, and the earliest BeFrugal and TopCashback take a claim. A pending purchase can then take several weeks to confirm, so pending isn't missing. Two clocks are running: the store has to report the sale, and then the return window has to close before the portal is paid. If the click shows in your portal's history but the cashback hasn't posted, the system saw you and the purchase is probably still pending. If the portal shows it as rejected or voided, that's a different problem, and a claim is the next step (what each clock actually is).

The part we can't promise

We haven't found a portal that publishes a tracking failure rate, and we can't work one out ourselves, because we don't see your orders. So "how often does this work?" has no real number attached to it, and any figure you see quoted is an estimate.

What we can say is that the causes are well understood and mostly avoidable, and that the failures cluster in a small set of situations: blockers, gift cards, outside codes, marketplace sellers inside a store's own site, and subscriptions. Here's the full diagnosis when something has already gone wrong, and the 60-second self-test walks the same list in the order worth checking.

So treat cashback as a strong likelihood rather than a certainty, and size your reliance accordingly. On a $40 order, click through and move on. On a $900 laptop, do the ritual above properly, because a few percent of $900 is worth five minutes.

What turns "lost" into "claimable"

A purchase that never tracks is not automatically money gone. Most portals have a missing-cashback claim form, and it approves or denies on evidence: the click, the order number, the total, the date, the confirmation. If you ran the ritual above, you have the order number, total, date and confirmation in one screenshot, and the click shows in your portal's history (or in our click receipt, if you were signed in), so filing is a form-filling exercise rather than an archaeology project.

There's a deadline on it, though. Portals count a claim window from the purchase date (BeFrugal's is 60 days, TopCashback's around 100 days, Rakuten publishes none), and a claim filed after it closes may not be reviewed. On BeFrugal that window overlaps its confirmation range, which runs to 60 days or more, so if a purchase still shows nothing late in that range, file before day 60 rather than waiting for it to confirm.

So: screenshot the confirmation, know your portal's window, and before the next big purchase, spend ten seconds checking what the store is actually paying today.