A landed cost purchase orders lets you assign a supplier's invoice as a landed cost to a stock order.

Raise a purchase order as you would a stock purchase order.  The important field which flags this PO is for landed costs is the PO to Attribute Costs dropdown.

From the dropdown, choose the PO number of the received PO that you want to attribute the total value of this PO to as landed costs.  NOTE only received stock POs are available in this list to attribute.

Click Add Product to add a line item to the PO as per any purchase order. This is usually a NonStock product such as shipping, customs, duty etc. It is recommended that you add your common landed costs to your product lists as non-stock items.

Check the totals match the supplier invoice and click Save and Approve to save the changes.  

If you already have the invoice from the landed cost supplier, from the landed cost purchase orders detail screen, click Receive Purchase Order.  Receive the purchase order as per normal, even if it is a non-stock line item.

Click Save when done.  The total value of this PO is now attributed to the line items on the attributed PO by qty and value.