Three systems, three numbers
Your supplier says they shipped 2,000 units. Your prep centre says they received 1,960 and have 380 left after sending shipments to Amazon. Seller Central shows 1,540 received across those shipments. Your spreadsheet, updated last week, says you have 450 at the prep centre. Nobody is sure which is right, and the next reorder depends on the answer.
When you ask the prep centre, they send an export. When you compare it with Amazon's receiving, the SKUs are named differently, some shipments are split, and some units were relabelled. It takes an afternoon to get close to an answer, and you still are not sure.
Where the numbers part company
Each system counts at a different moment, and units can change identity along the way, for example when a manufacturer barcode is covered with an FNSKU label or items are bundled.
| Point in the chain | Who counts | Common source of difference |
|---|---|---|
| Supplier dispatch | Supplier packing list | Short-shipped cartons |
| Prep centre receipt | Prep centre | Damaged on arrival, miscounted cartons |
| Prep and label | Prep centre | Rejects, bundles, relabelling |
| FBA shipment created | You | Planned versus packed differences |
| Amazon receiving | Fulfilment centre | Shortfalls, units received elsewhere |
When each step is recorded in a different system with different product codes, differences hide at the joins.
What disagreeing figures cost
You cannot claim a supplier shortfall if you only notice it months later. You cannot claim an Amazon receiving shortfall if you are not sure what was packed. You reorder based on a stock figure that may be wrong in either direction. And a prep centre that miscounts regularly goes unchallenged because nobody has clean numbers to challenge them with.
At year end, the stock valuation depends on the least reliable of the figures, and your accountant asks questions nobody can answer quickly.
Each of these gaps may be small. What makes them expensive is that they compound. A supplier short-ships a carton, the prep centre miscounts another, one shipment is received short at Amazon and one unit per hundred is rejected at prep. Nobody sees any single problem clearly, so none of them is followed up, and the stock you thought you had slowly turns out not to exist.
There is also a trust cost with your prep centre. Without clear figures, every disagreement turns into a vague complaint on your side and a defensive answer on theirs, and the relationship suffers without either side being able to prove anything.
The stock ledger we build
- A product code map linking supplier codes, your SKUs, FNSKUs and the prep centre's item codes, so a unit is recognised at every step.
- Supplier packing lists read from the documents you receive, recorded as expected stock.
- Prep centre receipts and stock levels read daily from their portal export or API, recorded as received and on hand.
- FBA shipments and their packed quantities recorded when created, and Amazon's received quantities read through the Selling Partner API.
- A ledger per SKU showing each movement, with a running balance at each location and any difference at each step highlighted.
- A weekly exceptions list: supplier shortfalls, prep centre discrepancies and Amazon receiving gaps, each with the documents needed to follow it up.
If you run your own warehouse instead of using a prep centre, the same ledger reads from your warehouse system or a stock sheet.
A week with the ledger
On Monday you open one screen and see stock at the supplier, in transit, at the prep centre and at Amazon for every SKU. Where a number disagrees between steps, the ledger shows which step and by how much. The exceptions list has three items: a carton short from the supplier, a prep centre count that does not match their own shipments out, and a small Amazon receiving gap still within the waiting period.
Your conversation with the prep centre changes. Instead of 'our numbers do not look right', it is 'you received 1,960 on the 4th and shipped 1,580 in three shipments, so on-hand should be 380, but your export shows 350'. Specific questions get specific answers.
Reordering improves because the stock figure you use is traceable and trusted.
Does this describe your stock records?
- Your prep centre's figures and your own never quite agree.
- Supplier shortfalls are spotted too late to claim.
- Product codes differ between supplier, prep centre and Amazon.
- Stock checks take an afternoon of spreadsheet matching.
- Your year-end stock figure is a best guess.