Estimated landed cost vs actual landed cost — what's the difference and when does each apply?

Two numbers, two jobs

Estimated landed cost (ELC) — a planning number stored on the SKU: what you expect a unit to cost all-in. Used before goods arrive — margin planning, valuing incoming stock, filling gaps when actuals don't exist yet.
Actual landed cost — calculated at receiving: PO price plus this receipt's allocated share of real extra costs. This is what your inventory is valued at and what COGS uses (FOB Cost vs. Landed Cost Explained).

Margin and pricing decisions made before receiving use the ELC — a stale ELC quietly misprices your catalog.
Month-end checks flag in-stock SKUs missing an ELC (see the Close the Books checks and What are zero cost sale items in the Close the Books report?).
A large gap between ELC and actuals is an early-warning signal: your freight/duty assumptions are out of date, or costs are not being entered on shipments.

Enter expected charges on POs/shipments up front — or automate them with cost rules (Automatically add freight, duty, and handling charges to purchase orders) — so actuals are complete.
Review SKUs where actual receipts consistently land above or below the ELC, and update the ELC.
Use the variance threshold setting to control when the system automatically overrides the actual cost with the ELC — if the gap between actual and ELC exceeds the threshold, the ELC is used in place of the cost-layer cost for sales items.

Surface	Number used
Inventory valuation, COGS	Actual landed cost (cost layers — Understanding Cost Layers in Inventory Management)
Not-yet-received stock, planning, margin previews	Estimated landed cost
Pushing costs to sales channels	Configurable — e.g. Shopify: Push Unit or Landed Cost Independently of Inventory