An transaction's total cost is returned in the AmountPaid property in GetItemTransactions or GetSellerTransactions API call.
NOTE.
1. If the given transaction has not yet been marked as paid by the item's seller, the AmountPaid equals to the TransactionPrice + the shipping cost defined in AddItem call (returned in ShippingServiceCost property).
2. If the given item's transaction has been paid (marked paid by the seller) then the AmountPaid equals to the TransactionPrice + the actual shipping cost ( ShippingServiceCost ) + the shipping insurance (if the buyer has selected it) + the tax (if applicable).