The Margin Trap Nobody Talks About
Free shipping is the most expensive decision most ecommerce brands make. Not because shipping costs money. Because it makes your Google Ads account lie to you.
When you absorb shipping into your product price or eat it entirely, your ROAS looks healthy. Your agency reports look clean. But your P&L tells a different story.
Let us walk through why.
The Maths That Breaks Your Account
Consider a product retailing at £45 with a £12 cost of goods. Your margin before shipping is £33. Looks strong.
Now add free shipping. Royal Mail Tracked 48 costs you £4.49 on a good day. Heavier items push that to £7 or more. Suddenly your margin is £26 to £28.50.
But here is where it gets dangerous. Google Ads does not know about your shipping cost. It optimises toward revenue. So when it reports a 4.0 ROAS on that £45 sale, it is calculating based on an £11.25 cost per acquisition.
Your actual contribution margin after CPA and shipping? Often under £15. On heavier items, sometimes under £5.
How Free Shipping Distorts Bidding
Smart Bidding uses conversion value to determine how aggressively to bid. If your conversion value is £45 but your true margin after shipping is £26, Google is overbidding by 42%.
This means:
- You are paying more per click than the economics justify
- Your target ROAS is set against the wrong baseline
- Products with high shipping costs get the same bid treatment as lightweight items
The result is systematic margin erosion that accelerates as you scale.
The Free Shipping Threshold Problem
Many brands use a free shipping threshold, say £50, to protect margins. But this creates a different problem in Google Ads.
Orders below the threshold include a shipping charge, which inflates the conversion value Google sees. Orders above it do not. This creates inconsistent signals that confuse Smart Bidding.
Google cannot distinguish between a £55 order with free shipping and a £42 order with £4.99 shipping reported as £46.99. Both look similar in value, but their margin profiles are completely different.
What to Do About It
1. Feed your true margin into Google Ads
Use Profit on Ad Spend (POAS) bidding. Strip out shipping costs, COGS, and VAT from your conversion value. Let Google optimise toward actual profit, not revenue.
2. Segment by shipping cost band
Group products by weight and shipping cost. Bid more conservatively on heavy items where free shipping eats the most margin.
3. Model the threshold impact
If you run a free shipping threshold, calculate the actual margin on orders just above and just below it. You may find that orders clustering around your threshold are your least profitable.
4. Consider shipping as a margin lever, not a marketing cost
Free shipping is not a Google Ads problem. It is a commercial decision that has Google Ads consequences. Your finance team and your PPC strategy need to be in the same conversation.
The Uncomfortable Truth
Most agencies never ask about your shipping costs. They optimise toward ROAS because that is what the platform measures. But ROAS without shipping data is a vanity metric.
If your agency cannot tell you the post-shipping margin on your top 20 SKUs, they are flying blind. And so are you.
Free shipping can be commercially sound. But only if your bidding strategy accounts for its true cost. Otherwise, you are subsidising delivery with your ad budget and calling it growth.