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Merge BigCommerce and Poshmark Sales Data for Quarterly Tax Estimates

Quarterly estimated tax payments depend on knowing real revenue across every channel, not just the platform that's easiest to check.

Why BigCommerce and Poshmark 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.
Poshmark: a sale date in the seller's local time; listing price and the Poshmark commission shown separately; an order number tied to a single sold item, since bundles are the main exception. Also, the export is aimed at individual resellers, so it has far fewer columns than the bigger platforms and no shipping/tax breakdown.

For quarterly tax estimates, 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 Poshmark 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.