The challenge with selling on multiple platforms is that Shopify, Amazon, and your POS system all track inventory their own way. If these don’t sync to a single source of truth, you end up with mismatched counts, incorrect cost of goods sold, and financial statements that don’t reflect reality.

SKU consistency is the foundation. Every product needs a single SKU that’s used everywhere. When the same product has different identifiers on different platforms, you can’t match sales back to inventory or track costs properly. Set up a master product list with one SKU per item and enforce it across Shopify, Amazon, your point of sale, and your accounting software.

Track purchase costs at the item level. When you buy inventory, record what you paid per unit including shipping and any fees to get the product to you. This landed cost is what you’ll use to calculate cost of goods sold. If you buy the same product at different prices over time, you need a consistent method. Weighted average cost works well for most e-commerce businesses because it smooths out price variations without requiring you to track specific units.

COGS timing matters more than most sellers realize. When you sell an item on Amazon, the cost should hit your books at the time of sale, not when Amazon deposits the funds two weeks later. If your accounting only captures costs when cash moves, your profit reports are wrong because revenue and costs are mismatched in time.

Reconcile platform reports to your accounting software monthly. Shopify gives you a payout report. Amazon gives you settlement reports. Your POS has its own sales reports. Each one captures fees, refunds, and settlements differently. Your books need to show gross sales, platform fees, net deposits, and the corresponding inventory cost for each sale. This reconciliation catches errors before they compound into a mess that takes hours to untangle.

Inventory accounting software becomes necessary once you’re selling meaningful volume across channels. Tools like Cin7, Skubana, or Stocky act as a central hub that syncs inventory counts and feeds data to your accounting software. Without this layer, you’re manually updating counts across platforms and hoping nothing oversells before you catch it.

The accounting integration is where things often break down. You might have perfect inventory tracking in your management software, but if it doesn’t flow correctly into QuickBooks, your financial statements are still wrong. Make sure the integration handles sales, cost of goods sold, and inventory adjustments properly.

If your books are already a mess from trying to manage multiple platforms without proper systems, bookkeeping cleanup and catch-up services can get you back to accurate records. The goal is knowing exactly what you have, what it cost, and what you’re making after platform fees across every channel.