If you own eCart, you can modify the Store order summary behavior on the confirm page to capture the transaction ID and store it in the orders table...
you would need to first add the column to the orders table, the add the binding for the paypal response to the binding table, exactly how to do this depends on what type of paypal transaction you are using: PayPal Direct payments or Paypal standard.
Then you would modify the store order summary to capture the transaction id.