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In some cases when I call GetItemTransactions or GetSellerTransactions, the value for ShippingServiceSelectedCost in a Transaction does not appear to include ShippingServiceAdditionalCost when the QuantityPurchased is greater than 1.
Why does it include this additional cost sometimes, and not include it other times?
The value for Transaction.Checkout.Details.ShippingServiceSelectedCost depends on the number of ShippingServices available for the item, and what state the Checkout process is in.
When there is only 1 shipping service available for an item, the value for Transaction.Checkout.Details.ShippingServiceSelectedCost always initially takes the ShippingServiceCost from that 1 shipping service.
If the user proceeds to go through Checkout, either completely through to payment, or just through the Buyer Requests Total Flow, the Transaction.Checkout.Details.ShippingServiceSelectedCost will be calculated using both ShippingServiceCost and ShippingServiceAdditionalCost (if QuantityPurchased is greater than 1).
If the buyer has not started the Checkout process, then the value for ShippingServiceSelectedCost will be equal to the ShippingServiceCost, regardless of the value of QuantityPurchsed.
The value for Transaction.Checkout.Status.IncompleteState generally indicates of the Buyer has started the Checkout process.
If the value is 1 (Checkout incomplete), this usually indicates that Checkout has not started, and additional costs will not be included in ShippingServiceSelectedCost.