If you sell online in Europe and your products show up in Google's Shopping ads, you are already using a Comparison Shopping Service — a CSS. By default it is Google's own, Google Shopping. Most merchants use it without ever noticing, because it is the option switched on automatically when a Merchant Center account is linked to Google Ads.
It is not the only one. Since 2017, third-party comparison sites have been allowed to run your product ads on your behalf, and a small industry has grown up around that. The question merchants ask is always the same: stay with Google, move to a third-party CSS, or run both?
What follows separates what Google actually documents from what vendors claim, and sets out the criteria that let you decide without kidding yourself.
The CSS programme came out of a penalty, not a product plan
On 27 June 2017 the European Commission fined Google €2.42 billion for abusing its dominant position: the search engine gave its own price comparison service prominence in its results, at the expense of rival comparison sites. The decision required Google to treat competing comparison services no less favourably than its own, and gave it 90 days to propose a remedy (European Commission press release IP/17/1784, 27 June 2017).
Google's answer was the CSS programme. The Shopping box was opened to third-party comparison sites, which can buy placements in it on behalf of merchants. In practice, the name of the CSS appears under the ad instead of "By Google". Google contested the decision to the last instance and lost: in a judgment of 10 September 2024 (Case C-48/22 P) the Court of Justice of the European Union dismissed the appeal and made the fine final (CJEU press release, 10 September 2024).
One point matters for a merchant: that judgment concerns the fine and the finding of abuse, not the validity of the remedy. It changed nothing in the day-to-day workings of the CSS programme.
Where the case stands in 2026
The dispute itself is not closed. On 23 July 2026 the European Commission fined Google €890 million under the Digital Markets Act: €460 million for self-preferencing in Google Search and €430 million for restrictions imposed on developers in Google Play. Among other things, the Commission objects to Google promoting its own services — shopping, hotels, transport, sports results — over third-party services, and orders it to bring the non-compliance to an end (European Commission announcement, 23 July 2026).
What that implies for the display of product results has not been settled publicly. Be wary of articles that already describe the future results page: nothing is documented by Google at this stage. What is documented is a change already in place — CSS ads for product pages. A comparison service in the programme can run a product carousel pointing to its own product page, competing with merchants' ads "on a non-discriminatory basis", with the landing page hosted on the CSS domain and showing offers from several merchants (Google CSS Center help).
One thing to flag to whoever maintains your feed: CSS management is moving into the Merchant API, where the Csses service is deprecated in favour of the Accounts service, and the older Content API for Shopping was shut down on 18 August 2026 (Google for Developers documentation). For a shop on a bespoke integration, that is maintenance, not strategy.
What choosing a CSS actually changes
Start with the official rules, the ones Google publishes. The programme covers 20 European countries, the United Kingdom included. You can send your product data to the CSS of your choice, Google Shopping among them, and you can use several CSSs at the same time. Above all, a merchant cannot bid against itself, however many CSSs represent it (Google Merchant Center help). That last point settles a common worry: running two CSSs does not push your own bids up against each other.
Then comes the commercial argument every third-party CSS leads with: roughly 20% off cost per click on Shopping campaigns. The reasoning is coherent — the CSS buys the placement for you and does not take the margin Google takes from itself — but Google documents no such rate anywhere in its official help. Treat that figure as a supplier claim to be verified against your own data, not as a property of the product.
| Criterion | Google Shopping (default CSS) | Third-party CSS |
|---|---|---|
| Setting up | Nothing to do, active by default | Merchant Center linked to the CSS, a few days |
| Cost | Included in the price of clicks | Flat fee, percentage of spend or commission on revenue, depending on the provider |
| Effect on CPC | The baseline you compare against | A saving claimed by the CSS, to be measured on your own campaigns |
| Control of accounts | Your accounts, your control | Varies: check who owns the Ads and Merchant Center accounts |
| Reversibility | Not applicable | Possible, but write the exit into the contract |
Questions to ask before you sign
A CSS is not a decorative badge. To stay in the programme, a comparison site has to meet conditions Google publishes: let users search for and compare products, show offers from at least 50 distinct merchant domains per country, offer sorting and filters, and not operate as a marketplace on its own domain. Sites that fall short can be suspended (CSS programme requirements, Google help). That filters out the more fanciful offers, though not the bad contracts. The due diligence is the same as for any search marketing provider: read the terms, not the pitch.
- Who owns the accounts? Your Google Ads account and your Merchant Center should stay yours. A CSS that insists on creating accounts in its own name makes you dependent on it.
- How is the service billed? Monthly fee, percentage of media budget, commission on revenue: compare the total cost, not only the saving announced on CPC.
- What commitment, and what exit? Ask in writing for the procedure to move back to Google Shopping, and how long it takes.
- Who runs the campaigns? Some CSSs only "carry" the ads; others manage bidding as well. Those are different jobs at different prices, and worth separating from your existing Google Ads management.
- What volume is involved? Below a few thousand pounds of Shopping budget a month, the potential saving is quickly diluted by management fees and migration time.
Measure before you decide, and after
Average CPC is a poor judge on its own: it also falls when you lose the most expensive auctions, which are usually the most competitive ones. Before any switch, freeze a baseline of at least four weeks and record cost per conversion, revenue generated, impression share and click volume, campaign by campaign. After the switch, compare the same indicators over an equivalent period, outside promotional weeks. That is the only way to know whether the announced saving turned into margin.
A single dashboard is enough — Looker Studio, or any tool wired to your campaigns — provided conversions are tracked correctly. That is the prerequisite worth checking before opening any debate about CSSs: if sales tracking is incomplete, no comparison will hold. On most accounts, this kind of measurement and campaign oversight has more effect than arbitrating between comparison services.
The position in one sentence: Google's CSS is perfectly fine while Shopping remains a secondary channel, and the subject becomes interesting once Shopping accounts for a significant share of your sales and your campaign structure is already clean. In that order. Before optimising a percentage of CPC, check that your search terms, your product feed and your product content are not leaving money on the table — long-tail queries and feed quality usually pay better. And nothing forces you to choose: testing a third-party CSS on part of the catalogue while keeping Google Shopping on the rest remains the most honest way to settle it.
Common questions
What is a Comparison Shopping Service?
A CSS is a price comparison site authorised by Google to run product ads in the Shopping box on behalf of merchants. The programme grew out of the European Commission decision of June 2017, which required Google to treat rival comparison services no less favourably than its own. Google Shopping is itself a CSS, switched on by default for any merchant who links Merchant Center to Google Ads.
Can you use several CSSs at the same time?
Yes. Google states in its Merchant Center help that a merchant can send product data to the CSS of its choice and use several of them simultaneously. It also states that a merchant cannot bid against itself, however many CSSs represent it, so combining them does not inflate your own bids.
Does a third-party CSS really lower cost per click?
Third-party CSSs commonly advertise a saving of around 20% on cost per click, but Google publishes no figure of that kind in its official documentation. The only reliable verdict comes from your own data: compare cost per conversion and revenue over two equivalent periods, not just average CPC, which also falls when you stop winning the most contested auctions.
Which countries does the CSS programme cover?
Google Merchant Center help lists 20 European countries, among them the United Kingdom, Germany, France, Spain, Italy, the Netherlands, Belgium and Switzerland. Outside that area the question does not arise: Shopping ads run there without an intermediary comparison service.
Did the 2024 Court of Justice judgment change anything for merchants?
Not directly. The judgment of 10 September 2024 dismissed Google's appeal and made the €2.42 billion fine final, but it concerns the penalty and the finding of abuse, not the operation of the CSS programme. Merchants had nothing to do as a result of that decision.
At what Shopping budget is a third-party CSS worth considering?
There is no official threshold, but the arithmetic is simple: the expected saving on clicks has to clearly exceed the CSS fees and the migration effort. Below a few thousand pounds of monthly Shopping spend the gap is often too small to justify the added complexity, especially as feed quality and campaign structure tend to deliver faster gains.
Is switching CSS reversible?
Moving back to Google Shopping is possible, but it is a contractual matter as much as a technical one. Ask for the exit procedure and its timescale in writing before you sign, and make sure the Google Ads and Merchant Center accounts stay in your name so that the return does not depend on a provider's goodwill.