Price comparison websites for online stores: the CPC model and Google's CSS rule

A price comparison website lists offers for the same product from different retailers side by side, and a store pays to appear in that listing — usually per click, sometimes a commission on a sale closed on the platform itself. For an online store, a price comparison website is another paid traffic channel alongside Google and Meta, with its own product data feed and its own payment model to maintain, much like a feed in Google Merchant Center. In 21 European countries, most of the EU among them, even Google's own product ads go through this kind of service: Google requires Shopping ads and free product listings there to be submitted via a Comparison Shopping Service (CSS). This article looks at how a price comparison website works from a retailer's side, when a pay-per-click listing actually pays for itself, and what EU law requires around customer reviews and discounted prices shown in a listing.

How a price comparison website works for a retailer

The mechanism is the same regardless of which site runs it: a retailer supplies data about its offers, the site lists those offers next to competitors' offers for the same product, and the shopper picks which store to buy from. Offers are usually submitted through the same kind of data feed used for Google Merchant Center — see our guide to the Merchant Center feed for what a feed needs to contain and how closely it has to match the retailer's own product pages.

In the model retailers meet most often, a shopper who picks a listed offer is redirected to the retailer's own product page and buys there — the comparison site is a traffic source, not the place the sale happens, and the retailer pays only when that click occurs (a pay-per-click, or CPC, model). Some comparison platforms instead let the shopper complete the purchase on the comparison site itself, with the retailer paying a commission on the order rather than for the click. Which model a given platform offers, and on what terms, is set out in that platform's own seller panel and price list — check it directly before committing budget, rather than assuming it matches a platform you've used elsewhere.

The CPC model: when does a click pay for itself

In a CPC listing, you pay for the click whether or not it turns into an order. Working out whether that's worth it needs only three numbers: the CPC rate the platform charges, your own conversion rate from that specific traffic, and the margin you earn on the order. The rate is set by the platform and the product category; conversion and margin are specific to your store, so don't substitute a market average for either.

The formula is simple:

cost per order = CPC rate ÷ conversion rate

A click pays for itself when that cost is at or below the margin you earn on a single order. The higher your conversion rate from that traffic, the lower the cost per order at the same CPC rate — and if the order margin is lower than the calculated cost, the channel loses money on every order, regardless of how much traffic it sends.

The same formula runs in reverse. If you know your order margin, it gives you the minimum conversion rate at which a given CPC rate pays off: CPC rate ÷ order margin. If the conversion rate from comparison-site traffic is lower than that, the channel loses money in the CPC model — and that is the quickest test to run before you start optimising offers at all.

Where a platform instead charges a commission on completed orders, the calculation is simpler: the cost per order is the commission rate multiplied by the order value, and the channel pays off when that amount sits below the order's margin — no conversion-rate variable is needed, because the risk of a click with no purchase sits with the platform, not the retailer. Compare it with the commission rate for your own category from the platform's price list, not with a headline "from" rate.

The two formulas can be set side by side. The commission model comes out cheaper than CPC when commission × order value is lower than CPC rate ÷ conversion rate. With a low conversion rate and an expensive click, the commission model wins; with a high conversion rate and large baskets, paying per click does. Where a platform offers a combined model, you don't have to choose one rule: clicks through to your store are billed per click, and orders completed on the platform by commission.

Customer reviews on a comparison listing, and what EU law requires

Reviews shown on a comparison site, or on the retailer's own page, fall under the EU's Unfair Commercial Practices Directive (2005/29/EC), as amended by the Omnibus Directive (EU) 2019/2161. Where a trader gives shoppers access to consumer reviews of a product, information on whether and how it ensures that published reviews come from consumers who actually used or bought that product counts as material information (Art. 7(6)) — leaving it out can make the practice a misleading omission.

The amended Directive also adds two practices to its blacklist (Annex I, points 23b and 23c), banned in all circumstances: stating that reviews come from consumers who actually used or bought the product without taking reasonable and proportionate steps to check this, and submitting, or commissioning someone else to submit, false reviews or endorsements, or misrepresenting reviews to promote a product. A tool that confirms a reviewer actually bought the product (a "verified purchase" badge, for instance) is one of those reasonable steps, but it doesn't replace telling shoppers how the check works — put that information where the reviews are shown, whether on a comparison site or in your own store.

Reviews also feed back into the break-even calculation. If they raise the conversion rate from the same click, they lower the cost per order at the same CPC rate. You can check this in your own data by comparing conversion before and after reviews are switched on.

Member states enforce this Directive through their own consumer-protection authorities, and the exact penalty regime varies by country — check the implementing law and the regulator responsible in each country you sell to.

Discounted prices in a comparison listing

A comparison listing shows a price next to the product name and photo, so it counts as advertising the product together with a price — the same rules that apply on your own product page apply there too. Under the EU's Price Indication Directive (98/6/EC), as amended by the Omnibus Directive (new Art. 6a), any announcement of a price reduction has to show the prior price, defined as the lowest price the trader applied during a period of not less than 30 days before the reduction — not an arbitrarily inflated "was" price. Member states may set different rules for goods that perish or expire quickly, a shorter period for products on the market for less than 30 days, and a special rule for progressively increased reductions, so check how the country whose consumers you sell to has transposed it.

In practice, if your feed sends a reduced price with a "was" price to a comparison listing, that prior price has to meet the same 30-day test as the one on your own product page, and both have to match — not a figure that only becomes correct once the shopper clicks through. A gap between the feed price, the comparison-listing price and the product-page price is a deeper feed-matching problem too, one we cover from the Merchant Center side in the feed-configuration article.

Google Shopping and the CSS requirement

In the 21 countries Google lists — 18 EU member states (Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Poland, Portugal, Romania, Slovakia, Spain and Sweden) plus Norway, Switzerland and the UK — Google's Shopping ads and free product listings don't go straight from the retailer to the search results page; they're placed through a Comparison Shopping Service (CSS). Google's own documentation states that CSSs "must be used" for this in those countries. CSS providers differ in what they do for a retailer: "Some CSSs manage product data and campaigns on behalf of the merchant, while others provide tools allowing merchants to manage their setup themselves." A retailer can submit its data to more than one CSS at once, and Google Shopping itself operates as one CSS among others, bidding on behalf of the merchants it represents like any other CSS — and the ad shows which CSS uploaded the offer, in the "By CSS" link at the bottom.

The programme traces back to a European Commission antitrust decision of 27 June 2017: the Commission fined Google €2.42 billion under Article 102 TFEU for giving an illegal advantage to its own comparison shopping service, and required Google to apply the same processes and methods to rival comparison shopping services as to its own. That decision was an EU enforcement act: it never bound Switzerland, and the UK has since left the EU — yet both are on Google's country list for the programme today.

The practical consequence for a retailer: if you run Google Shopping ads, your offers reach them through a CSS — by default Google Shopping itself, or a third-party CSS you choose instead. It isn't an alternative to Google's product ads; in the countries on Google's list, it's part of how they're served. We cover the campaign side of this — including Performance Max and target ROAS — in our Google Shopping article.

Other price comparison sites in Europe

Beyond Google's CSS programme, independent comparison engines operate in many European markets — idealo, for example, runs separate country sites (idealo.de, .fr, .it, .es and others), each with its own retailer terms. We don't quote coverage, traffic or retailer numbers for any of them here, because none is published in a form we could verify — treat them as examples of the category, not as one EU-wide service with a single price list. The commercial terms, feed requirements and ranking factors for any comparison site are set in that platform's own seller panel; check them directly for the platform and category you're considering, rather than assuming they match a competitor's.

Checklist before you connect a product feed

Before you launch or increase budget on a comparison site, check:

• Do the titles, photos and descriptions in your feed match what the shopper sees after clicking through? A mismatch raises the cost per order by pulling down the conversion rate from that traffic — the mechanism behind the break-even formula above. For how to write product copy that holds up for both search and conversion, see our article on product descriptions.
• Is the price identical across the feed, the comparison listing and your product page — and if you're showing a reduction, do you have on record the lowest price you applied in the 30 days before the cut?
• Does the delivery cost shown with the offer match what the shopper sees in the cart? A mismatch a shopper only discovers at checkout costs you the order anyway.
• Is a review-verification mechanism switched on and correctly configured, and is the disclosure about how you verify reviews actually visible — not just technically present?
• Is the same product feed you submit to the comparison site also valid against Google Merchant Center's own requirements? If you're running both channels, see our Merchant Center configuration guide so you're not maintaining two feeds that drift apart.
• Does the payment model you're signing up for — CPC, commission, or a mix some platforms offer — suit the margin and conversion rate in that product category? Check this before you agree to the platform's terms, not after your first invoice.

Only once you've checked all of that is it worth deciding which payment model makes sense for your assortment and margin.