Channel margins compare like for like only when every channel is reduced to the same contribution line: net revenue to the seller, minus marketplace fees, minus fulfillment cost, minus attributable ad cost, minus the cost of returns, minus FIFO cost of goods. Each marketplace names those pieces differently, computes them on a different base, and sometimes bundles two of them together. An AI compares channel margins by building that line for each channel from reconciled transactions and then reading it. It cannot compare what was never normalized, and neither can you.
The marketplace's cut. Amazon calls it a referral fee, a percentage of the total price by category; sell.amazon.com's pricing page lists Pet Supplies at 15 percent with a $0.30 minimum. Walmart also calls it a referral fee, and marketplace.walmart.com's pricing page computes it on a total that includes shipping and handling. eBay calls it a final value fee, and ebay.com's selling fees page computes it on the total amount of the sale, sales tax included, plus a per-order fee. TikTok Shop takes a platform commission, and on affiliate-driven orders the creator's commission comes out too. Shopify takes none of these; the cost that fills the slot is payment processing.
Fulfillment. FBA and WFS charge a fulfillment fee per unit by size and weight, deducted in the settlement. A seller-fulfilled order on eBay, Shopify, or TikTok Shop costs a shipping label on a carrier account, which arrives on a separate invoice. Same economic line, two different documents.
Ads. Amazon Ads and Walmart Connect run cost-per-click campaigns billed to the account. TikTok Shop's GMV Max campaigns are billed as ad spend, and per the TikTok help center every paid and organic order for a product in the campaign is attributed to the campaign while it runs. Affiliate commissions are a per-order deduction that behaves like a variable selling cost, not an ad budget. A brand campaign on paid social that lifts three channels at once belongs to none of them.
Returns. An Amazon refund reverses most of the referral fee. eBay's fee credits policy governs what comes back on a refund. A Shopify return costs a second label. A TikTok Shop return on an affiliate order may or may not claw back the commission. Return rates also differ by channel for the same product.
The multichannel profit attribution glossary defines the terms. The point here is the method.
Illustrative numbers. Take a ceramic dog bowl set, landed FIFO cost $9.20, priced at $29.99 on Amazon (FBA), on the seller's Shopify store (seller-fulfilled), and on TikTok Shop (seller-fulfilled through the platform's label service). July.
Amazon, 350 units
Shopify, 140 units
TikTok Shop, 260 units
Same product, same price, same cost basis: 20.3 percent, 16.7 percent, 7.5 percent. The reasons are legible once the line is normalized. TikTok Shop carries two selling costs where Amazon carries one (commission plus affiliate commission, 14.3 percent of revenue combined, against Amazon's 15 percent referral fee), a label that costs more than FBA's fee, ad spend at 14.7 percent of revenue against Amazon's 9.3 percent, and a return rate of 8.1 percent of units against Amazon's 3.4 percent. Shopify's margin is dragged by the label and by a $720 campaign that may not belong to it at all.
Crunch, the analytics AI inside ConnectBooks, reads reconciled data from Amazon, Shopify, Walmart, TikTok Shop, and eBay and answers questions about margin, fees, advertising, and returns by finding what changed, comparing the periods you name, isolating the products responsible, explaining why, and recommending a next step. Asked "why is the dog bowl margin so much lower on TikTok Shop," it returns the four reasons above with the dollars behind each, because each fee type posted to its own account, labels were tagged to the channel, refunds tied back to orders, and the same $9.20 cost applied everywhere.
ConnectBooks builds that base through settlement and payout reconciliation into QuickBooks or Xero at the transaction level, FIFO COGS per unit, and profit reporting by SKU and by channel. The comparison report puts two channels or two periods side by side on that same line, which is the manual version of the question.
The $720 paid social campaign is the honest limit. It ran for the brand. Some of it sent people to the Shopify store, some sent them to search "ceramic dog bowl" on Amazon, and some reached people who bought on TikTok Shop from a creator's video a week later. No AI on ledger data can allocate it with any basis in fact. It can show you contribution before brand spend by channel and brand spend as one line underneath, and it can tell you that spreading the $720 across all three channels by revenue share would put Shopify at 30.7 percent, Amazon at 17.1 percent, and TikTok Shop at 4.3 percent. It cannot tell you that allocation is right. It is an assumption dressed as a split.
The GMV Max attribution is the second limit. Because TikTok credits organic orders to the campaign while it runs, the console's return figure overstates what the ad spend caused. The ledger knows spend and total TikTok sales; it does not know which orders were incremental.
The third is affiliate content. A creator's video keeps selling after the commission window closes and after the campaign ends. The ledger sees units; it does not see the video.
Compare channel margins on the normalized line, keep shared spend unallocated, and treat any tool's cross-channel allocation as a model, not a measurement.
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