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Merge Walmart Marketplace and Mercari Sales Data for Multi-Store Reporting

Reporting on multiple storefronts only works cleanly once every store's export uses the same column layout.

Why Walmart Marketplace and Mercari exports don't line up

Walmart Marketplace: an Order Date column separate from the ship-by date; item price and a separate Walmart referral fee; a numeric Purchase Order ID. Also, each order line can appear as its own row with a repeated order ID, similar to Shopify's line-item export.
Mercari: a sold date in the seller's local time; item price and Mercari's selling fee shown separately, with shipping often bundled into the price; an order ID tied to a single sold item. Also, like other reseller apps, the export is minimal with no tax breakdown and mixes cancelled orders into the same file.

For multi-store reporting, that mismatch is exactly what causes hours of manual cleanup every period.

Open CSV Experts and set your target columns once (Date, Channel, Order ID, Total).
Drop in your Walmart Marketplace export and map its columns once.
Drop in your Mercari export and map it the same way — both merge into one sheet, normalized.
Export as CSV or Excel. Same-layout files merge automatically next time.

No. Everything runs in your browser using JavaScript — your files are never sent to a server.

Yes. Add as many source files as you want; each distinct file layout is mapped once and remembered.