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Can the amount of money that a buyer has to pay be calculated or determined through the API?
The simple answer is that the amount of money that a buyer has to pay cannot be calculated through the API.
When a purchase is made for one item on eBay, a transaction is created for that purchase.
The details of the transaction are returned in the response of the GetItemTransactions (GIT) and GetSellerTransactions (GST) API calls.
The amount of money that the buyer will have to pay includes the actual price of the item, any sales tax the seller is charging, the shipping and handling and insurance costs, and any other charges or credits that the seller indicates.
These line items that make up the amount of money that the buyer will have to pay are not known immediately at the time the purchase is made.
They become known once the buyer proceeds through the eBay Checkout flow.
The page where the data becomes known in the eBay Checkout flow is commonly known as "Review Your Purchase(s)".
Therefore, if the GIT and GST calls are made before the buyer has gone through the eBay Checkout flow, the total amount of money cannot be determined.
Here are the data points that are available, and a description of each data point with regards to the validity of the data: