A problem hidden in plain sight
Most travelers trust that when they click a link on a travel blog, comparison website or deal page, they’re being guided toward the best available price. After all, the industry standard claim has always been: “We earn a small commission at no extra cost to you.”
But in many cases, that claim is no longer true.
During recent research, I discovered something that seriously undermines consumer trust: travelers who book a rental car through an affiliate link can pay 20 to 30 percent more than those who visit the same website directly and/or via incognito browsers.
No warning, no transparency, no explanation.
Yet the price difference is real – and significant.
A real example: 29.55% price difference
To verify what was happening, I conducted a full test booking using an affiliate link from TradeTracker to Rentcars.com.
Same dates, same location, same rental company, same car class.
But the prices told a different story.
Direct booking (incognito browser, no cookies): €301.31 for 21 days with cashback.
Booking via affiliate link: €427.69 for the exact same rental – + ~48 euro higher base price and no cashback.
That’s a markup of €126.38, equal to 29.55%, purely because the user entered the site through an affiliate link. Consumers do not see this. Publishers do not see this. But the system does.
Why this is a serious problem
Misleading pricing practices
Direct visitors often see cashback offers, seasonal discounts or special promotions.
Users who enter via affiliate links often do not.
Two users.
One website.
Two completely different prices.
Consumers overpay without realizing it
Travelers trust travel blogs and comparison sites to help them navigate confusing markets. Instead, they can end up paying significantly more without ever knowing why.
Publishers lose integrity
Content creators and independent publishers work hard to provide honest recommendations. But when platforms inflate prices for affiliate traffic behind the scenes, publishers unknowingly promote more expensive options that harm their audience.
This damages credibility, authority and trust – and publishers have zero control over it.
Millions lost due to broken tracking
The affiliate industry already struggles with unreliable tracking:
GA4 missing large portions of traffic, aggressive last-click hijacking from deal sites and invisible deduplication behind the scenes.
These problems cost publishers millions every year.
Now consumers are being pulled into the same problem.
This is not an isolated case
Rentcars is not the only platform where this happens.
We have repeatedly seen similar patterns with Booking . com, especially when users land on specific hotel pages through affiliate links.
Prices increase, discounts disappear and cashback offers vanish.
And nobody informs the consumer.
A call for transparency and regulation
This situation should not be possible under modern consumer protection laws.
Yet the affiliate ecosystem operates in a grey zone where major platforms can alter pricing strategies without oversight, leaving both consumers and publishers exposed.
It’s time for a proper investigation.
We call on the Consumentenbond, AVROTROS Radar and Charlotte’s Law & Fine Prints.
Consumers deserve honest pricing.
Publishers deserve fair compensation.
The industry needs transparency, not hidden markups.
Test it yourself
You can reproduce the findings within seconds.
Affiliate link naar deze deal: https://www.rentcars.com/nl-nl/booking/list/4318-1768395600-4318-1770210000-0-0-0-0-0-0-0-0?utm_source=tradetracker&utm_medium=tradetracker_nl&utm_campaign=425599&requestorid=4352
Schone link naar deze deal (wel even in incognito browser openen): https://www.rentcars.com/nl-nl/booking/list/4318-1768395600-4318-1770210000-0-0-0-0-0-0-0-0?
1. Open this TradeTracker → Rentcars link
(It will show higher prices for many users.)
https://www.rentcars.com/nl-nl/booking/list/4318-1768395600-4318-1770210000-0-0-0-0-0-0-0-0?utm_source=tradetracker&utm_medium=tradetracker_nl&utm_campaign=425599&requestorid=4352
2. Open an incognito window and use this clean link
(No cookies, no affiliate tracking.)
–> https://www.rentcars.com/nl-nl/booking/list/4318-1768395600-4318-1770210000-0-0-0-0-0-0-0-0
Two different realities – on the same website.
What immediately stands out is that the affiliate links use a different sorting method, showing more expensive deals by default and removing all cashback offers. When you sort both links by lowest price, the differences become even bigger. Through affiliate links, the best deals and cashback options are completely hidden – a classic example of ‘dark patterns’ that mislead consumers and operate right on, or over, the edge of legality.
Final thought
This is not a small glitch or an innocent technical flaw.
It is a structural issue that disadvantages the very people who rely on travel blogs, consumer platforms and comparison websites for honest recommendations.
As long as these practices continue, I will continue to expose them.
If you work in travel, affiliate marketing or consumer protection, test it yourself – and help put an end to a system that harms both consumers and publishers.
