We use PayPal to process our payments when you buy from us. PayPal issues a unique transaction ID to uniquely identify and refer to your transaction whenever you make a payment. This transaction ID is sent to you via email in the receipt that PayPal sends you.
Our system also sends you an email with the transaction ID in it. You downloaded the product from this email.
The email that our system sends looks similar to the following (you can get the transaction ID from there):