A retailer's checklist before partnering with a CSS

Why due diligence matters here

The Comparison Shopping Service market in the UK has grown steadily since the EU's 2017 antitrust ruling opened Google Shopping to third-party competitors. That growth has attracted serious operators — and a few that are less serious. The difference between a good CSS partner and a poor one is not always obvious from a sales deck.

The good news: verifying the fundamentals takes about thirty minutes. Here are eight things to check before you sign anything.

1. Verify the CSS is genuinely registered with Google

This is the most basic check and the one most often skipped. A legitimate CSS must be registered with Google as a Comparison Shopping Service and must appear in the CSS Centre. Any CSS partner should be able to provide their CSS domain and confirm their registration status.

What to do: Ask the CSS partner for their registered CSS domain. You can cross-reference this with Google's publicly available information. If they cannot provide this, or if they hesitate, that is a significant red flag.

A registered CSS has a direct technical integration with Google's Shopping infrastructure. An unregistered entity claiming to offer CSS services is, by definition, not operating a CSS — they may be reselling through another party, which introduces unnecessary complexity and cost.

2. Understand the commercial model

CSS partners operate under different commercial arrangements. The three most common are:

• CPA (cost per acquisition): You pay a commission only when a sale occurs. This is the lowest-risk model for retailers.
• CPC (cost per click): You pay for each click, regardless of whether it converts. This shifts risk towards the retailer.
• Monthly fee + CPC/CPA hybrid: A fixed retainer plus a performance component.

What to do: Get the commercial model in writing before any technical integration begins. If the model is CPA, confirm what counts as a qualifying sale — is it based on last-click attribution, post-view, or something else? Confirm the commission rate, any tiered structures, and whether the rate applies to gross or net revenue.

A pure CPA model through an affiliate network (Awin, CJ, Tradedoubler, etc.) is the most transparent arrangement because the network handles tracking, validation, and payment.

3. Check the contract terms

CSS contracts should be straightforward. You are granting a partner the right to place your products in Google Shopping through their CSS and paying them a commission or fee for doing so. The contract should not require exclusivity, should not lock you into long minimum terms, and should not restrict your ability to run your own Shopping campaigns.

What to do: Read the contract — specifically the termination clause, the exclusivity clause (there should not be one), and any minimum spend commitments. A thirty-day rolling notice period is reasonable. A twelve-month lock-in with penalties is not.

Also check who owns the data. Your product feed data and performance data should remain yours. The CSS partner should be able to provide reporting but should not claim ownership of your catalogue information.

4. Confirm which affiliate networks they support

If you are working on a CPA basis, the CSS partner will typically track conversions through an affiliate network. You need to confirm that they operate on the same network where you already have an affiliate programme — or be prepared to set one up.

What to do: Ask which networks the CSS supports. The major ones in the UK are Awin, CJ Affiliate, Rakuten Advertising, Tradedoubler, and impact.com. If you already have an active programme on one of these, integration is usually quick — often just a matter of approving the CSS as a publisher on your existing programme.

If the CSS uses a proprietary tracking system rather than an established network, proceed with caution. Proprietary tracking means less transparency and more potential for disputes over attribution.

5. Ask about reporting and transparency

You should have visibility into what the CSS partner is doing with your products. At a minimum, you need to see: which products are being bid on, the impressions and clicks generated, the cost per click the CSS is paying, and the resulting conversions and revenue.

What to do: Ask for a sample report or a walkthrough of their reporting dashboard before you commit. Check that reporting is available at the product level, not just in aggregate. Aggregate-only reporting makes it difficult to assess whether the CSS is genuinely adding incremental value or simply claiming credit for traffic you would have won anyway.

Good CSS partners will also provide data on where their clicks differ from your in-house campaigns — showing coverage gaps and incremental queries rather than just total numbers.

6. Understand cannibalisation protections

Google's auction mechanics prevent a merchant from competing against itself (the deduplication rule ensures only the highest bid from a given merchant enters any auction). But beyond the auction mechanics, you should understand how the CSS partner approaches bidding on products where you already have strong in-house campaigns.

What to do: Ask the CSS partner how they handle overlap with your existing campaigns. Specifically:

• Do they bid on all products in your feed, or do they focus on gaps?
• Can you exclude specific products or categories from their bidding?
• Will they reduce bids on products where your in-house campaigns are already dominant?

A CSS partner that is financially motivated to be efficient (particularly one on a CPA model) will naturally avoid wasting budget on products where you already win most auctions. But it is worth confirming their approach explicitly.

7. Check country coverage

If you sell across multiple European markets, confirm which countries the CSS partner covers. CSS operates across the European Economic Area, but not all CSS partners have the infrastructure or expertise to manage campaigns in every market.

What to do: Ask for a list of countries where the CSS is active and has a track record. If you only sell in the UK, this is less relevant — but if you have ambitions to expand into Germany, France, or the Nordics, choosing a CSS partner with existing coverage in those markets saves you from having to find a new partner later.

Also check whether the CSS partner can handle multi-currency feeds and localised product data if you sell cross-border.

8. Ask for references

This is the simplest and most effective check. A CSS partner with a genuine track record will be able to provide references from retailers of a similar size and sector.

What to do: Ask for two or three references and actually contact them. The questions to ask are straightforward:

• How long have you worked with this CSS partner?
• What incremental revenue have they generated?
• How is the reporting and communication?
• Have you experienced any issues with cannibalisation or attribution?
• Would you recommend them?

If a CSS partner cannot provide references, or if the references are all from very small retailers when you are a mid-market business, take that as a signal.

Putting it together

None of these checks requires technical expertise or a significant time investment. A thirty-minute call with the CSS partner, combined with a careful read of the contract, covers most of the ground.

The CSS market is mature enough that there are several credible operators in the UK. The difference between a good partnership and a frustrating one usually comes down to transparency, commercial alignment, and the willingness of the CSS partner to prove their value with data rather than promises.

Do the due diligence upfront. It is far easier than unpicking a bad partnership six months in.