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Merge BigCommerce and Mercari Sales Data for Monthly Sales Reporting

A monthly sales report across channels needs one date format and one revenue column, not a per-platform patchwork.

Why BigCommerce and Mercari exports don't line up

BigCommerce: an Order Date column that includes a full timestamp; Subtotal and Grand Total both included, which are easy to mix up when mapping; a numeric Order ID. Also, many optional columns are exported blank depending on store configuration, which can throw off auto-mapping by header position.
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 monthly sales 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 BigCommerce 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.