Why Overselling Happens (and Why It's Dangerous)
Picture this: you have 5 units of a popular product left. Someone buys 3 on Amazon at 10:02 AM. At 10:04 AM, someone buys 4 on Shopify. You now owe 7 units to customers but only have 5 in the warehouse. That's overselling, and it sets off a chain of ugly consequences.
On Amazon, overselling triggers cancellations, which directly damages your Order Defect Rate. Get above a 1% defect rate and Amazon starts throttling your listings. Keep it up and you'll face a suspension. On Shopify, you're refunding a customer who thought they made a purchase — they're not coming back.
This problem barely exists when you sell on one channel. The moment you add a second — Shopify plus Amazon, or Amazon plus eBay, or any combination — inventory sync becomes the single most critical operational concern in your business.
The root cause is simple: when a sale happens on one platform, there's a delay before the other platform knows about it. That delay can be 15 seconds with a good sync tool, or 15 minutes with a bad one, or 4 hours if you're relying on Shopify's native Amazon integration. During that window, both platforms are showing stale inventory numbers.
How Inventory Sync Actually Works
Every sync tool operates on the same basic principle: it sits between your sales channels and your inventory source of truth, pushing updated stock levels whenever something changes.
There are two sync methods:
Push-based sync: When a sale happens on Amazon, the sync tool detects it and pushes updated inventory to Shopify (and vice versa). This is faster but depends on how quickly the tool receives sale notifications.
Poll-based sync: The tool checks each platform on a set interval (every 5, 10, or 15 minutes) and reconciles the numbers. This is slower but more reliable for catching manual adjustments.
Most modern tools use a combination: push for immediate sales and poll as a safety net to catch anything that slipped through.
Sync frequency is everything. A tool that syncs every 15 minutes creates 15-minute windows for overselling. A tool that syncs within 60 seconds shrinks that window to almost nothing. For high-velocity products (10+ sales per day), even a 5-minute sync delay is dangerous.
Shopify's Native Amazon Channel: Why It's Not Enough
Shopify has a built-in Amazon sales channel that handles basic inventory sync. It's free, it's easy to set up, and for many sellers, it's the starting point.
The problems:
- Sync speed: Updates can take 15 minutes to several hours depending on Amazon's API throttling
- Limited to simple products: Bundles, kits, and multi-pack inventory don't work well
- One-directional limitations: Changes made directly in Amazon (like FBA restocks) may not reflect accurately
- No warehouse management: It's purely channel-to-channel sync with no concept of physical warehouse locations
- Fragile connection: The integration breaks more often than sellers would like, requiring manual reconnection
For a seller doing under 20 orders per day on a single product line, Shopify's native integration might be fine. For anyone beyond that, a dedicated tool is worth the investment.
Dedicated Sync Tools: The Real Options
Ordoro ($59/month and up) is built specifically for multichannel inventory management. It syncs inventory between Shopify, Amazon, eBay, Walmart, and others with near-real-time updates. What sets Ordoro apart is its purchase order management and supplier tracking — you can manage inventory from purchase to sale in one place. It handles bundles and kits natively, meaning if you sell a "starter kit" that includes items A, B, and C, selling one kit deducts the components from all channels.
Veeqo (free, owned by Amazon) syncs inventory across Shopify, Amazon, eBay, and others. The Amazon ownership gives it a slight edge on Amazon integration speed. Since Veeqo is free, it's hard to argue against at least testing it. The trade-off is that it's a newer platform and some features feel less polished than established competitors.
SKULabs ($299/month and up for multichannel) is the heavy-duty option for sellers with complex warehouse operations. It combines inventory sync with warehouse management, barcode scanning, and pick/pack workflows. Overkill for small sellers, but essential for operations managing 5,000+ SKUs across multiple channels and warehouse locations.
ShipStation ($9.99/month and up) includes inventory management that's good enough for basic multichannel sync. It tracks stock across connected channels and adjusts quantities when orders ship. It's not as robust as Ordoro or SKULabs for pure inventory management, but if you're already using ShipStation for shipping, adding inventory sync means one fewer tool to manage.
SellerCloud (custom pricing) is an enterprise-grade option for sellers doing high volume across many channels. It handles complex scenarios like virtual kits, dropshipping inventory, and multi-warehouse allocation rules. If you're doing $1M+ in revenue across 5+ channels, this is the tier of tool you need.
Safety Stock: Your Insurance Policy
No sync tool is perfect. Network delays, API rate limits, server hiccups — there will always be moments when your inventory counts are slightly off. Safety stock buffers protect you.
The concept is simple: if you have 50 units in the warehouse, tell your channels you have 45. That 5-unit buffer absorbs any sync lag.
How much buffer to hold depends on your velocity:
- Slow movers (1-2 sales/day): 2-3 units buffer
- Medium movers (5-10 sales/day): 5-10 units buffer
- Fast movers (20+ sales/day): 10-15% of available stock as buffer
Most sync tools let you set this as a global rule or per-SKU. Set it and review it monthly. If you're never touching the buffer, you might be holding too much back and leaving sales on the table.
Handling FBA vs. FBM Inventory
If you use Amazon FBA for some or all of your Amazon orders, inventory gets more complicated. FBA inventory lives in Amazon's warehouses. Your Shopify inventory lives in yours. These are physically different pools of stock.
Two approaches:
Separate pools: FBA inventory is only for Amazon. Your warehouse inventory is only for Shopify. Simple to manage but inefficient — you might have 100 units sitting in FBA while your Shopify store shows out of stock.
Unified pool with allocation: Your total inventory is 200 units. You send 100 to FBA and keep 100 in your warehouse. Your sync tool needs to understand that FBA stock is reserved for Amazon and warehouse stock is available for all channels. When FBA stock runs low, you send more from your warehouse.
Most tools handle this by treating FBA as a separate "warehouse location." Veeqo and Ordoro both support this model, letting you allocate stock across FBA and your own warehouse while maintaining accurate channel-level inventory.
Bundles and Kits: The Inventory Nightmare
Bundles break basic inventory sync. If you sell shampoo individually ($12) and as a 3-pack bundle ($30), selling one bundle needs to deduct 3 units from the individual shampoo count. Selling one individual shampoo needs to check whether enough stock remains for pending bundle orders.
Shopify doesn't handle this natively. You need a tool with component-level inventory tracking — Ordoro, SKULabs, and SellerCloud all support this. Without it, you'll oversell bundles constantly.
The setup: define each bundle as a virtual SKU with component SKUs and quantities. When the bundle sells, the system deducts the components. When a component sells individually, the system recalculates how many bundles can still be fulfilled.
Low Stock Alerts and Reorder Points
Stockouts are the other side of the overselling coin. You can't sell what you don't have, and every day out of stock is lost revenue and damaged search rankings (especially on Amazon, where stockouts tank your organic ranking).
Set up alerts at two thresholds:
- Warning level: enough stock for 2 weeks of sales at current velocity — time to reorder
- Critical level: enough stock for 3-5 days — expedite your restock or temporarily pause advertising
Most sync tools and inventory management platforms include alerting. ShipStation has basic low-stock alerts. Ordoro ties alerts to reorder points and can auto-generate purchase orders. SKULabs calculates reorder points based on sales velocity and lead time.
Dealing With Sync Delays in the Real World
Even with the best tools, sync isn't instantaneous. Here's how to mitigate real-world delays:
Run flash sales carefully. If you're promoting a product on Shopify with a 50% off sale, your Amazon inventory will briefly show availability for stock that's already sold on Shopify. Either pause the Amazon listing during the sale or increase your safety stock buffer temporarily.
Watch for API throttling. Amazon throttles API calls during peak periods (Prime Day, Black Friday). Your sync might slow from 60 seconds to 10 minutes during these events. Increase safety buffers a week before major sale events.
Audit weekly. Run a manual inventory count against what your tools show at least once a week. Drift happens — items damaged in the warehouse, miscounts during receiving, returns processed incorrectly. Catching drift early prevents compounding errors.
Have a cancellation playbook. When overselling does happen (and it will, eventually), have a plan. Contact the customer immediately, offer a discount on a substitute product or a credit for future purchase, and process the cancellation before the customer even notices. Speed here is everything.