What Is a Comparison Shopping Service (CSS)?

A Comparison Shopping Service, or CSS, is a platform certified by Google to submit product listings into Google Shopping auctions on behalf of merchants. Since 2017, every retailer advertising on Google Shopping in the European Economic Area does so through a CSS, whether they realise it or not.

If you have ever run Shopping ads in Europe, your products were submitted by a CSS. By default, that CSS is Google Shopping Europe (GSE), a separate business unit within Google. But you are not limited to GSE. You can choose any certified CSS partner, and that choice has a direct impact on what you pay per click.

This guide explains what a CSS is, how the system came into existence, the different types of CSS providers available today, and what this all means for your advertising budget. If you are already familiar with the basics, you can skip ahead to the EU antitrust ruling or read about the 20% CPC advantage directly.

The History Behind CSS

To understand CSS, you need to understand the problem it was designed to solve. The story starts long before 2017.

In 2002, Google launched Froogle, a free product search engine. It was rebranded as Google Product Search in 2007, and merchants could list their products at no cost. Then, in 2012, Google made a significant change: it transformed the service into Google Shopping, a paid advertising platform. Merchants now had to pay for placement through product listing ads (PLAs).

This shift created an immediate problem. Google was operating both as the marketplace (running the Shopping ad auction) and as a competitor within that marketplace (promoting its own Google Shopping results over rival comparison sites in general search results). Established comparison shopping services like Kelkoo, PriceRunner, and Foundem saw their traffic collapse as Google gave its own Shopping box prime placement at the top of search results.

These companies filed complaints with the European Commission. After a seven-year investigation, the EC ruled in June 2017 that Google had abused its dominant market position. The ruling required Google to give rival comparison shopping services equal access to the Shopping auction. Google's response was to create the CSS partner program, which opened the auction to independent CSS providers.

Types of CSS Providers

The CSS landscape has evolved considerably since the program launched in September 2017. Today, there are several distinct categories of CSS providers, each with a different business model and value proposition.

Legacy Comparison Sites

These are the companies that existed before the 2017 ruling as traditional price comparison websites. They were the original complainants whose fight with Google led to the CSS program in the first place. Examples include Kelkoo, PriceRunner, Idealo, and LeGuide. These platforms transitioned their existing comparison shopping infrastructure to also serve as CSS providers under the new rules. They typically combine their price comparison functionality with CSS services, offering merchants both organic visibility on their comparison platform and access to the Google Shopping auction at reduced CPCs.

Technology Platforms

This category includes CSS providers that were built specifically for the CSS opportunity. Companies like Cobiro, Producthero (now part of Channable), and BigShopper fall into this group. These platforms focus on making the CSS switch as simple as possible. Cobiro, for instance, provides a self-service platform where merchants can switch their Merchant Center to Cobiro CSS in minutes and immediately benefit from reduced CPCs without changing anything about their campaigns. Technology platforms typically charge a flat monthly fee rather than taking a commission, which makes the cost structure transparent and predictable.

Affiliate CSS

Shoparize is the most prominent example of the affiliate model. Instead of charging a subscription or flat fee, affiliate CSS providers take a commission on sales generated through the Shopping ads they submit. This model can be attractive for merchants who want to avoid upfront costs, but it means the CSS takes a percentage of your revenue rather than a predictable fixed amount. For merchants with high order values or strong conversion rates, the commission model can end up costing more than a flat-fee provider.

Managed Service

Providers like Bidnamic bundle CSS with full bid management and campaign optimisation services. Rather than simply offering the CPC advantage, they take over the management of your Shopping campaigns entirely. This is suited for merchants who want a hands-off approach, though it typically comes at a higher total cost.

Whitelabel CSS

Companies such as Adstrong, Label Up, and Moose enable digital marketing agencies to run their own branded CSS. Instead of reselling another provider's CSS service, the agency gets its own CSS identity with its own branding. This means the agency's name (or a brand of their choice) appears in the "By" label on Shopping ads. Whitelabel CSS is primarily a B2B product for agencies that manage multiple merchant accounts. Cobiro also offers a whitelabel CSS solution for agencies that want to operate under their own brand.

The "By [CSS Name]" Label

When your Shopping ads appear on Google, a small text label appears beneath them reading "By [CSS Name]." If you are using the default Google Shopping Europe, this label says "By Google." If you switch to an independent CSS partner, it will read "By Cobiro," "By Kelkoo," or whatever the name of your chosen CSS is.

This label was part of Google's remedy to give users transparency about which CSS is submitting the listing. In practice, the label is small and most shoppers do not notice it. Multiple studies and split tests have shown no measurable impact of the CSS label on click-through rates or conversion rates. Shoppers are focused on the product image, price, and merchant name, not on the tiny attribution text at the bottom of the ad.

That said, some merchants initially worry about the label. If you are concerned, you can run a test: operate two Merchant Centers side by side, one on GSE and one on an independent CSS, and compare performance. You will almost certainly find no meaningful difference in CTR, but you will see a clear difference in CPC.

Geographic Scope

The CSS program applies to the 21 EEA countries, plus the United Kingdom and Switzerland. This is because the European Commission's ruling applies within the European Economic Area, and Google extended the program to the UK (post-Brexit) and Switzerland voluntarily.

The full list of eligible countries is: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, and the United Kingdom.

If you advertise on Google Shopping in any of these countries, you are eligible to use an independent CSS partner. If you only advertise outside of these countries (for example, in the United States or Australia), the CSS program does not apply and there is no alternative to GSE.

What CSS Does Not Change

There are persistent misconceptions about what switching CSS actually does. Let us set the record straight.

CSS Does Not Change Your Campaigns

Your Google Ads campaigns, ad groups, product groups, bidding strategies, and budgets remain completely untouched when you switch CSS. You continue to manage everything in your own Google Ads account exactly as before. The CSS switch happens at the Merchant Center level, not the Google Ads level.

CSS Does Not Affect Your Product Data

Your product feed, product titles, descriptions, images, pricing, and availability data are all unaffected. The CSS does not touch your product data in any way. Your feed continues to flow into your Merchant Center exactly as it did before.

CSS Does Not Give You Different Ad Placements

Your Shopping ads appear in exactly the same places regardless of which CSS you use. The same Shopping carousel, the same Shopping tab, the same surfaces across Google. Independent CSS partners have identical placement access to GSE. This was a core requirement of the European Commission's ruling.

CSS Changes the Cost Structure of the Auction

This is the only thing CSS changes, and it is the thing that matters. When Google Shopping Europe submits a bid on your behalf, it applies an approximately 20% margin. When an independent CSS submits the same bid, that margin is not applied. The result: your bid goes further, you pay less per click, or both. For a deeper look at how this works mechanically, read the 20% CPC advantage explained.

How Merchants Choose a CSS

The process of choosing a CSS partner is straightforward, but there are a few factors worth considering. Cost is the most obvious: some providers charge monthly fees, others charge per click, and others take commissions. Transparency matters too. You want a provider that makes it clear exactly what you are paying and what you are getting.

Ease of switching is another important factor. With a provider like Cobiro, the switch takes minutes and requires no technical changes to your campaigns or feed. Some providers require more involved onboarding, which may or may not be justified depending on the additional services they offer.

Finally, consider lock-in. Reputable CSS providers do not lock you in. You should be able to switch away just as easily as you switched in. If a provider requires long-term contracts or makes it difficult to leave, that is a red flag. For a comprehensive guide to evaluating CSS partners, see our choosing a CSS partner page.

Next Steps

Now that you understand what a CSS is, the next article in this series covers the EU antitrust ruling that created the CSS program. Understanding the legal and regulatory background helps explain why the 20% margin exists and why it is likely to persist for the foreseeable future.