Handling inventory across multiple warehouses comes down to one discipline: treat every location as its own ledger, and account for movement between them as a transaction rather than as a rounding difference. Sellers who skip that step end up with a single blended average cost, a company wide quantity that matches nothing on any shelf, and a cost of goods sold figure that cannot be traced to a specific fulfillment event. The fix is procedural and takes about a week to implement properly. Here is the sequence.
Step 1: Define what counts as a location
A location is any place where you hold title to stock and where a unit could be picked. For most multi channel sellers that means at least four categories: Amazon fulfillment centers, a third party logistics warehouse, your own space, and goods in transit.
Amazon complicates this because its network moves your units between facilities without producing anything you would recognize as a transaction. For accounting purposes, treat the entire Amazon network as one location. Trying to mirror individual fulfillment centers produces enormous detail and no usable information.
Write the list down and do not add to it casually. Every location you create is a reconciliation you now owe every month.
Step 2: Choose where costing happens
This is the decision most sellers get wrong, and it is worth slowing down on.
If you apply a single weighted average cost across the whole company, a unit shipped from your 3PL and a unit shipped from Amazon carry identical cost, even when one arrived on a container at $4.10 and the other at $5.35 after a freight increase. Your margin by channel becomes fiction, because channel and location correlate.
Applying FIFO at the fulfillment location solves this. The unit that ships is costed against the oldest layer at the location it shipped from. This is what a proper multi location inventory layer does, and it is the specific thing ConnectStock, the inventory module inside ConnectBooks, was built to handle for marketplace sellers running stock across Amazon, a 3PL and their own warehouse.
Whichever you pick, pick once. Changing inventory costing methods generally requires filing for a change in accounting method with the IRS, which is not something to do casually mid year. Talk to your accountant before you change anything.
Step 3: Account for transfers, including freight
A transfer between your own locations is not a sale and produces no revenue. What it does produce is a change in cost basis, because moving pallets costs money.
The correct treatment is to roll inbound freight into the cost of the units at the receiving location. If you ship 500 units from your warehouse to an Amazon fulfillment center and the shipment costs $312 in freight, those 500 units now carry an additional $0.624 each at the Amazon location. Expensing the freight separately understates inventory on the balance sheet and overstates operating expense in the month of the transfer.
The exception worth flagging: if your locations sit in different legal entities, an inter location move may be an intercompany sale requiring elimination on consolidation. That is a different problem and one to raise with your accountant rather than solve in software.
Step 4: Book in transit as its own asset
Goods on a container are your inventory the moment title passes, which depends on the incoterms in your purchase order rather than on arrival date. Under FOB origin terms, title passes at the supplier’s dock.
Give in transit inventory its own balance sheet account. Two reasons. It keeps your on hand quantity honest, because in transit units cannot be picked and should not appear in availability. And it makes the quarter end count possible, since you can reconcile physical count to the on hand account without chasing what is on water.
Step 5: Reconcile per location
A company wide reconciliation that ties in aggregate can hide two offsetting errors. If your 3PL is 40 units short and your Amazon count is 40 units long, the total is perfect and both numbers are wrong.
Run the comparison location by location, monthly, on a fixed date. For each location, compare system on hand to the warehouse’s own report. Investigate anything above a tolerance you set in advance, usually a percentage of units rather than an absolute count, because a five unit variance means something different on 200 units than on 20,000.
A worked example
A seller carries one SKU across three locations at the start of March.
Own warehouse: 1,200 units, two receipt layers, 700 at $4.10 and 500 at $4.55. Amazon: 850 units, all at $4.55 plus $0.62 of inbound freight, so $5.17. Third party logistics: 400 units at $4.10 plus $0.38 freight, so $4.48. In transit: 2,000 units at $4.72, arriving March 18.
Total on hand is 2,450 units. Total inventory value is $4,872 plus $4,395 plus $1,792, which is $11,059, with a further $9,440 sitting in the in transit account.
During March the seller sells 600 units from Amazon and 150 from the 3PL. Under FIFO at the fulfillment location, Amazon COGS is 600 times $5.17, or $3,102. The 3PL COGS is 150 times $4.48, or $672. Total COGS is $3,774.
Run the same month on a company wide weighted average of $4.51 and COGS would be $3,383. The difference is $391 on a single SKU in a single month, and it runs in the direction of overstating profit on the channel that costs more to serve. Multiply across a catalog and the Amazon channel looks healthier than it is.
Step 6: Set reorder points per location
A company wide reorder point is not actionable, because you cannot order into “the company.” You order into a specific location with a specific lead time.
Amazon replenishment lead time is not the same as your supplier lead time, and the Amazon leg has its own receiving delay. Calculate each location’s reorder point from that location’s demand rate and that location’s total lead time, then hold safety stock against demand variability rather than against a flat percentage.
One cost that makes this more urgent than it looks: Amazon’s ageing inventory surcharge begins at 181 days and escalates from there, reaching $6.90 per cubic foot or $0.30 per unit, whichever is greater, once units pass 366 days. Overstocking a fulfillment center is not a neutral decision. Amazon publishes the current schedule on its Seller Central fee documentation, which is publicly readable without a login.
What to do first
If you are starting from a single blended cost and a spreadsheet, do steps one, four and five before anything else. Defining locations, separating in transit, and reconciling per location will surface most of the errors hiding in your current numbers. Costing method and reorder points are refinements that only pay off once the counts are trustworthy.
The Small Business Administration publishes general guidance on inventory and working capital, and the IRS small business pages cover the accounting method rules that constrain the costing decision. Neither substitutes for an accountant who has seen your purchase orders.
