Low-inventory-level fees need a days-of-supply model

Amazon's low-inventory-level fee changed the job of replenishment planning. A seller can be profitable at the old fulfillment fee, then quietly become unprofitable when a fast-moving SKU spends too many days below the supply threshold. The operational mistake is treating the fee as a finance line item. It starts as an inventory signal.

Amazon has explained the fee around historical days of supply: short-term and long-term inventory positions are compared with historical demand, and the fee applies when both are below the threshold Amazon sets for the program. That means the operator's daily question is not "how many units do we have?" It is "how many days can this ASIN sell at recent velocity before Amazon starts pricing scarcity into fulfillment?"

Build the model at parent-product level

Track units available, inbound units by shipment state, recent shipped units, and the parent product relationship. Parent-product aggregation matters because variants can hide the true risk. One slow child and one fast child can average into a misleading view if the replenishment decision is made only at SKU level.

Use three bands:

• Healthy: more than the threshold plus receiving buffer.
• Watch: above the threshold, but only if inbound arrives on time.
• At risk: below the threshold or projected below it before the next receiving window.

The receiving buffer should be conservative. If partnered-carrier pickup, FC appointment, and check-in usually take 18 days, a 28-day threshold is not a 28-day reorder point. It is closer to a 46-day operating target for manually replenished products.

Decide what not to replenish

Not every SKU deserves more FBA inventory. Wholesale and arbitrage sellers often carry opportunistic inventory that cannot be reordered. Private-label sellers may choose to sell through a seasonal run and accept the last few fee-bearing units. The point of the model is not to blindly ship more stock. It is to show where a fee will turn a sale into a bad sale.

When a SKU is at risk, compare four actions: replenish FBA, switch part of demand to FBM, raise price to absorb the fee, or stop chasing the last units. That decision belongs in margin review, not only in the warehouse queue.

What to review every Monday

Start the week with a ranked list: projected days of supply, expected fee exposure, contribution margin after the fee, inbound ETA, and owner. If the expected fee exposure is less than the cost of rushing a shipment, do nothing. If the fee flips contribution margin negative, act before the next sales spike.

The low-inventory-level fee rewards sellers who run replenishment like a margin system. Days of supply is the shared metric finance, purchasing, and warehouse teams can all act on.

Turn the signal into a replenishment rule

The model becomes useful when it changes the buying rule. For each replenishable product, define the minimum days of supply that must be available when a purchase order is placed, when units reach the warehouse, and when units are expected to be sellable in FBA. Those are three different dates, and treating them as one date is how teams end up paying scarcity fees even though they technically reordered on time.

The rule should also account for operational capacity. If the warehouse can prep only a certain number of units per day, inbound stock sitting at the door does not protect the ASIN yet. If Amazon receiving is inconsistent for a marketplace or fulfillment center pattern, add that receiving variance to the buffer. A low-inventory fee forecast that ignores prep and receiving is too optimistic.

Keep accepted exceptions visible

Some SKUs will intentionally run below the threshold. That is fine as long as the decision is explicit. A discontinued SKU, a seasonal tail, or a product with weak contribution margin may not deserve another inbound shipment. Mark those SKUs as accepted exceptions with an expiration date and a reason.

Without that note, finance will keep rediscovering the same fee and operations will keep treating it as a replenishment failure. With the note, the weekly review can separate real process misses from deliberate sell-through choices. The goal is not zero fees at any cost. The goal is knowing which fees are cheaper than the alternatives and which ones are avoidable margin leaks.