How CarParts.com drove incremental revenue with Fospha's always-on measurement

The Challenge

Before working with Fospha, CarParts.com relied heavily on Last Click attribution - giving disproportionate credit to lower-funnel, conversion-focused activity. In practice, this meant 95% of their Paid Social budget was concentrated in TikTok, with just 5% going to Meta - a split driven by historical convention rather than measured performance.

This created a structural bias: when teams can't trust the numbers, they default to demand capture channels that convert last, not channels that create demand. For CarParts, the result was a channel mix that looked defensible on paper but left significant growth on the table.

The team also faced growing misalignment between what platform reporting and GA4 were telling them - a common symptom of a fragmented measurement stack where ad platforms over-attribute to themselves and GA4 systematically under-credits demand generation. With no consistent, trusted source of truth, they lacked the confidence to invest in newer or higher-funnel channels.

The Solution

CarParts.com onboarded with Fospha in January 2025. From day one, they had access to two years of modeled performance data, giving them a long-range view of how every channel contributed to growth. Fospha's modeling also fully integrated with their custom ecommerce platform - something no other provider had been able to do.

At the core of Fospha is a Bayesian MMM, built to model the cross-channel impact of each marketing input on revenue - across every stage of the funnel. It measures not just what drives clicks or conversions, but the unseen value of impressions - the brand activity that drives future demand across all destinations, including Amazon and TikTok Shop.

Using Beam, Fospha's forecasting tool, CarParts.com identified a significant opportunity in Meta's Advantage+ campaigns. Saturation curves revealed they could scale investment by 50% and still beat their paid media ROAS target by 128%, a clear signal that Meta had incremental headroom and was underutilized.

Rather than relying on a single test, CarParts.com followed a structured learning agenda. Over the course of the partnership, they ran four Fospha-informed tests: two cross-channel budget reallocations, one optimisation goal test, and a Meta Conversion Lift Study. Each test was designed to validate a specific Fospha insight before committing to a larger budget shift - science without paralysis, action without blind faith.

When results came in, they were directionally consistent across every measurement approach - Fospha's model, GA4, and Meta's own reporting all confirmed the revenue increase. That cross-model agreement gave the team the confidence to act decisively.

To validate this, the team launched a Meta Conversion Lift Study (CLS) from May to June 2025. The results confirmed what Fospha had already shown: Meta was driving strong incremental returns and reaching net-new audiences efficiently.

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The Results

Armed with confidence from both Fospha's daily insights and the CLS findings, CarParts.com scaled Meta investment by over 200% YoY in June 2025.

Fospha recommended a single-cell, account-wide CLS due to the model identifying that all Meta segments were exceeding efficiency targets - highlighting the breadth of opportunity.

This reallocation drove measurable gains:

• Meta became the most efficient digital channel, delivering the highest ROAS
• 68% of Meta conversions came from new customers, helping CarParts reach fresh audiences

CarParts.com also began generating a greater number of new customer conversions YoY, indicating healthy expansion of their customer base.

This validated the team's hypothesis: Meta aligned with their audience and was their most incremental channel - but they needed the right mix of measurement tools to prove it.

The Rebalancing Story

The impact of shifting budget from TikTok to Meta is best understood through a before-and-after lens.

In the prior year, CarParts.com spent $130k on Paid Social - 95% allocated to TikTok, 5% to Meta. This year, guided by Fospha's insight that Meta's ROAS significantly outperformed TikTok's, they scaled total Paid Social investment to $452k and split spend evenly: 50% TikTok, 50% Meta.

The counterfactual is telling. Had CarParts maintained the original 95/5 split at current ROAS levels, that spend would have delivered an estimated $3.8M in revenue. The 50/50 rebalancing delivered $5.2M - an incremental $1.4M driven entirely by reallocating $231k toward the channel Fospha identified as having greater headroom.

For CarParts.com, one of Fospha's most underrated benefits was internal. With conflicting signals from GA4, Meta, and their own platform data, justifying budget decisions to leadership was a recurring challenge. Fospha provided a single, reconciled view that marketing and finance could interrogate together - making it easier to move budget upfunnel and defend those decisions with confidence.

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