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GetSellerTransactions doesn’t seem to return PayPal fees in Sandbox. Is this a Sandbox limitation?
Fees are returned in the GetSellerTransactions response, once the transaction's eBayPaymentStatus goes to NoPaymentFailure. For the ebayPaymentStatus to move to this state, the seller must list the item with the PayPal email address that is linked with their Sandbox account, else the status remains in PayPalPaymentInProcess and the fee field returns 0.