I'm using PayPal Checkout to process a transaction, as it has the look and feel I need for the customer. It returns an orderID, paymentToken, paymentID, etc but it does NOT offer a transaction ID. I record those entries. Now if I use the REST API on the same app, to issue a refund, and pass the paymentID (or any other I am returned), I get an error that it doesn't exist...because I need a transaction ID. SO, how do I use a paymentID (or orderID) and get a transactionID??
