Amazon FBA Inventory Management: How to Avoid Stockouts and Storage Fees (2025)

How Amazon FBA inventory works

With Fulfilment by Amazon (FBA), you ship your products to Amazon's UK fulfilment centres (warehouses). Amazon receives the stock, stores it on their shelves, and takes care of picking, packing, and shipping every order to your customers. Amazon also handles customer service and returns for FBA orders.

From your side, inventory management for FBA involves:

• Creating shipment plans in Seller Central to send stock to Amazon
• Labelling units with FNSKUs (Amazon's tracking barcodes) before sending
• Monitoring your available, reserved, and inbound quantities
• Deciding when to replenish and how many units to send
• Keeping your overall inventory health metrics in a good range

Seller Central shows your FBA inventory in the Manage Inventory section and with more detail in Inventory → Inventory Dashboard. Key metrics include days of supply remaining at your current sales rate, units reserved, and unfulfillable units (damaged or otherwise unsellable).

ASIN and FNSKU explained

Understanding Amazon's identifier system is essential for managing FBA inventory correctly.

An ASIN (Amazon Standard Identification Number)is a 10-character alphanumeric code that identifies a product in Amazon's catalogue. Every product listed on Amazon.co.uk has an ASIN. If you are selling an existing branded product, there will already be an ASIN for it — you list against it rather than creating a new one. If you are selling a new or private-label product, Amazon creates a new ASIN when you list it.

An FNSKU (Fulfilment Network SKU) is different. It is the barcode Amazon uses to track your specific units within their warehouse network. When you create an FBA shipment, Amazon generates an FNSKU label for each product. You print these labels and apply them to your units (covering the manufacturer barcode) before shipping to Amazon. This allows Amazon to attribute sold units to your seller account specifically, which matters when multiple sellers have stock of the same ASIN in the same warehouse.

Your Seller SKU is separate from both. This is your own internal identifier — the same one you use in Shopify, eBay, and your own records. You enter it when you create your FBA listing. Keeping your Seller SKU consistent with the SKU you use everywhere else is what makes multi-channel inventory sync possible.

Amazon's IPI score and why it matters

The Inventory Performance Index (IPI) is Amazon's measure of how efficiently you manage your FBA inventory. It is a score from 0 to 1000, updated weekly. Amazon considers anything above 450 to be healthy. Sellers with a score below 400 may have their FBA storage capacity limited.

The IPI is calculated from four factors:

• Excess inventory rate — the percentage of your FBA units that have more than 90 days of supply at your current sales rate. High excess inventory pulls your IPI down and increases your storage fees.
• Sell-through rate — how quickly your units sell relative to the average inventory you held over the past 90 days. A higher sell-through rate improves your IPI.
• Stranded inventory rate — the percentage of your FBA units that are in the warehouse but cannot be sold because the associated listing is inactive or has an error. Stranded inventory should be zero.
• In-stock rate for top products — how often your best-selling products are actually available. Frequent stockouts on your highest-velocity items hurt your IPI.

The practical lesson: Amazon rewards sellers who send the right quantity at the right time. Too much stock sitting idle is penalised. Too little (frequent stockouts) is also penalised. The goal is a lean, well-managed inventory that turns over regularly.

UK FBA storage fees — 2025 rates

Amazon charges two types of storage fees for UK FBA sellers:

Monthly storage fees

Charged based on the cubic metres of space your stock occupies in Amazon's UK fulfilment centres. Rates are higher in October–December (peak season). As a rough guide, standard-size products cost approximately £0.51 per cubic foot per month (January–September) and approximately £1.21 per cubic foot (October–December). Always check Amazon's current fee schedule in Seller Central as rates are updated periodically.

Long-term storage fees (Aged Inventory Surcharge)

Units that have been in Amazon's UK fulfilment centres for more than 365 days are charged an aged inventory surcharge. This is charged on top of the regular monthly storage fee and can quickly make slow-moving products uneconomical to hold at FBA. Amazon runs an inventory age report that shows you exactly which units are at risk.

How to avoid stockouts on Amazon FBA

Running out of stock on Amazon FBA is one of the most damaging things that can happen to an Amazon seller. When you go out of stock:

• You lose sales directly
• Your product's BSR (Best Seller Rank) drops, often dramatically
• Recovering your ranking takes time and often requires expensive PPC spend
• Competitors gain the customers you should have had
• Your IPI score falls due to a low in-stock rate

The best way to avoid FBA stockouts is to have a clear, documented reorder process:

1. Know your daily sales rate for each ASIN (find this in Seller Central's Sales Dashboard or Business Reports)
2. Know your supplier lead time — not just the typical time, but the worst case
3. Know Amazon's inbound processing time for your shipment type (typically 2–7 business days in the UK)
4. Add a safety buffer on top of that
5. Calculate your reorder point: (daily sales rate × (supplier lead time + Amazon processing time + safety buffer in days))
6. Set a trigger — when your FBA available quantity drops to this level, create a shipment

How to calculate your FBA reorder point

Here is a worked example. Suppose you sell 8 units per day of a product. Your supplier in Guangzhou takes 25 days to produce and deliver to your UK address. You then need 5 days to prep and ship to Amazon, and Amazon takes 5 days to process your inbound. Your safety buffer is 7 days.

Total lead time = 25 + 5 + 5 + 7 = 42 days

Reorder point = 8 × 42 = 336 units

When your FBA available quantity drops to 336 units, trigger a new shipment. If you wait until you are lower than this, you risk a stockout during your lead time.

For a more detailed breakdown, see our guide on how to calculate your reorder point.

FBA vs FBM: comparison table

FBM (Fulfilment by Merchant) means you handle all storage and shipping yourself. Both models have a place depending on your product and business. Here is how they compare:

Many experienced UK sellers use a hybrid approach: FBA for their fast-moving, standard-size products where Prime delivery makes a real difference to conversion, and FBM for heavy, large, or slow-moving products where FBA storage fees would erode margins.

Managing FBA alongside Shopify and eBay

This is where FBA inventory management gets genuinely complex — and where most UK multi-channel sellers run into problems.

If you are selling the same products on Amazon FBA and on Shopify and on eBay, you need to answer a critical question: which stock do those three channels draw from?

Option A: You hold all stock at Amazon and use Multi-Channel Fulfilment (MCF) — Amazon fulfils your Shopify and eBay orders too. This is simple in terms of stock management but adds cost per order and means Amazon branding on your Shopify parcels.

Option B: You hold separate stock for FBA (at Amazon) and for your own warehouse (to fulfil Shopify and eBay orders). This is the most common approach for established sellers. You maintain two separate pools of stock, and your inventory tool manages each one.

Option C: You use a 3PL that fulfils your Shopify and eBay orders while also prepping and shipping to Amazon FBA. In this case, all your physical stock is at the 3PL.

Regardless of which model you use, you need a central inventory tool that knows about all stock locations and can push accurate quantities to each channel's listings. Salyncconnects Amazon, Shopify, and eBay and can handle both your FBA quantities (pulling from Amazon's available FBA stock) and your own warehouse stock for non-FBA channels.

Stranded inventory

Stranded inventory is stock sitting in an Amazon fulfilment centre that is not actively listed for sale. This happens when:

• A listing is accidentally closed or deleted
• A listing has a pricing error that puts it below Amazon's minimum price floor
• A product fails a safety or compliance review
• You created an FBA shipment but never created the corresponding listing
• The ASIN was merged or split by Amazon

Stranded inventory accrues storage fees but generates zero sales. Amazon shows you stranded inventory in Inventory → Fix Stranded Inventory in Seller Central. You should check this report at least weekly and resolve any stranded units immediately — either by fixing the listing or creating a removal order.

Removal orders

When you have stock at Amazon that you want back or want destroyed, you create a removal order. Amazon will either ship the units back to your registered address or dispose of them (for a small fee). Reasons to use removal orders:

• Stock approaching the 365-day aged inventory surcharge threshold
• Unsellable (unfulfillable) units you want to inspect or refurbish
• Closing an ASIN permanently
• Receiving inventory back to fulfil on another channel (eBay, Shopify) yourself

Be aware that Amazon's removal processing time can be 2–4 weeks during peak periods. Plan ahead if you need stock returned before the aged inventory fee date.

How Salync connects Amazon to your other channels

Salync connects your Amazon Seller Central account (including your FBA inventory) with Shopify, eBay, and Etsy. The connection works in both directions:

• When a sale happens on Amazon, Salync deducts from your central count and updates your eBay and Shopify stock immediately
• When a sale happens on eBay or Shopify, Salync updates your Amazon listing quantity
• You can set per-channel buffers so Amazon always shows slightly lower stock than your actual FBA quantity, protecting against last-second simultaneous sales

Salync reads your available FBA quantity from Amazon's API — the live number Amazon shows as fulfillable. It does not count reserved, inbound, or unfulfillable units in your sync quantity, so customers on other channels never see inflated stock that isn't actually ready to ship.