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Why are some of my fixed price items showing with Free additional shipping?
Listing fixed price items are usually done with a Quantity greater than one.
In general, the shipping cost for fixed price items are specified by using the ShippingServiceCost and ShippingServiceAdditionalCost fields for each Shipping Service in the AddItem call.
The ShippingServiceAdditionalCost is the incremental cost for shipping each quantity purchased over one
If a quantity of one is purchased, the shipping charged is the ShippingServiceCost value.
If a quantity greater than 1 is purchased, the shipping charged is ShippingServiceCost + ShippingServiceAdditionalCost * (Quantity purchased -1).
When a fixed price item is listed with <Quantity>1</Quantity> then the ShippingServiceAdditionalCost specified in the AddItem call is discarded completely. This was done because there is no additional quantity and therefore no additional shipping cost to be concerned with for purchases on an item with Quantity = 1.
The problem happens when the item listed with a Quantity of one is revised with a Quantity greater than one:
As a best practice, specify the entire ShippingDetails container which includes the ShippingServiceAdditionalCost if you revise the Quantity from 1.