Half the supplement and beauty brands we talk to want the same thing: sell on Amazon and their own Shopify store without constantly manually updating stock in two places. The pitch for Shopify's Amazon integration (or any of the third-party sync apps) sounds simple: one source of truth, auto-updating stock, no overselling.
In practice, we've cleaned up roughly a dozen botched syncs in the past year. The pattern is almost always the same: a brand enabled sync without configuring the four things that matter, and 10 days later their best SKUs showed as "out of stock" on both channels while 400 units sat in Amazon FBA. This post is the exact setup we use to avoid that.
Decide which channel is the source of truth first
This is the single most important decision and the one most brands skip. Before you install any sync app, answer: when inventory counts disagree, which channel wins?
Three valid answers, each with different tradeoffs:
- Shopify is source of truth. Best when the majority of your stock sits in your own 3PL or warehouse and only a slice goes to FBA. Your Shopify inventory is the "real" number; Amazon receives a push.
- Amazon FBA is source of truth. Best when FBA holds most of your stock and Shopify orders ship from the same FBA inventory via Multi-Channel Fulfillment. Shopify pulls the available quantity from Amazon.
- Split allocation. You reserve a specific quantity for each channel. Harder to run but prevents overselling at the channel level.
Pick one before you install anything. Writing it down matters; we've seen two brands silently "both" because they didn't decide, and the sync fought itself for weeks.
The native Shopify Amazon integration vs third-party apps
Three real options:
1. Shopify's built-in Amazon integration
Available in the Shopify Admin under Settings → Apps and sales channels → Add Amazon. Free. Supports inventory sync, pricing sync and basic order creation.
Good for: brands that are already selling the exact same SKU on both channels and want the simplest possible setup.
Not great for: FBA fulfillment of Shopify orders (requires Multi-Channel Fulfillment configured separately), bundle SKUs that don't map 1:1 across channels, or stores with large variant catalogs.
2. Shopify's Amazon Multi-Channel Fulfillment (MCF) integration
Specifically for the "let FBA ship my Shopify orders" use case. Separate from the inventory channel. Works well. Setup takes about 30 minutes once you have your Amazon seller account properly linked.
3. Third-party sync apps (Codisto, Zapiet, Trunk, Sellbrite)
Pay to play, usually $29–$299/month depending on SKU count and features. Worth it if:
- You sell bundles that need to decrement multiple component SKUs
- You have variant mismatches between channels (3-pack on Amazon, 1-pack on Shopify)
- You sell on more than two channels
Not worth it if you have a simple 1:1 SKU mapping and modest volume.
The four traps that break orders
Trap 1: SKU mismatches
Your Amazon SKU and your Shopify SKU need to match exactly. Not "close". The sync key is the SKU string, including case, spaces and hyphens. "SLEEP-30" and "sleep-30" are different products. We've seen a brand lose a week of inventory updates to one lowercase letter.
Before enabling sync: export both channels' SKU lists, diff them, and fix mismatches first.
Trap 2: Bundle math
If you sell a 3-pack on Amazon that corresponds to 3 units of your Shopify 1-pack, the native Shopify integration can't do that math. Every Amazon 3-pack sale will only decrement your Shopify stock by 1. You'll silently oversell.
Fix: either sell matching 3-packs on both channels, or use a sync app that supports bundle component decrementing.
Trap 3: Reserved stock timing
Shopify holds inventory during checkout for about 10 minutes. Amazon holds it longer. When the two systems both try to "reserve" the same unit for different carts, one channel oversells. Mitigation: run with a 5–10 unit safety buffer per SKU. Not glamorous but it works.
Trap 4: Push timing
The native Shopify integration pushes inventory updates every 15–60 minutes, not real-time. A busy launch day can oversell inside that window. Third-party apps update faster (1–5 minutes). If you run flash sales or TikTok Live, use a third-party sync app or increase your safety buffer.
The setup we actually run on a supplement store
For most of our clients, the setup is:
- Shopify is source of truth. Even if FBA holds the majority of stock, Shopify's inventory system is cleaner to work with.
- Native Amazon integration for inventory sync. Free, reliable, enough for sub-$500k/year brands.
- Shopify MCF for order fulfillment. Shopify orders ship from Amazon's FBA stock automatically.
- 5-unit safety buffer per SKU. Prevents overselling in the 15-minute push window.
- Weekly manual audit for the first month. Compare Shopify's reported stock vs Amazon's actual FBA quantity. Catches configuration mistakes before they compound.
Past $500k/year or multi-channel (TikTok Shop, Walmart, etc.), move to a third-party sync app with real-time updates.
Before you flip the switch
Three things to do the week before you turn sync on:
- Export SKU lists from both channels, diff them, fix mismatches
- Decide and document which channel is source of truth
- Reduce each SKU's available quantity by 5 units as a safety buffer
If this feels like a lot, it is. Multichannel inventory is one of the places where a weekend of careful setup saves months of fire-fighting. If you want it done right the first time, tell us about your store. Amazon & TikTok Shop sync is included in every Brand Store we ship.