If you are using the payment method PayFlow Pro Hosted and are seeing this error after paying for a transaction on the hosted payment page, it means that the settings in your Paypal Manager account need to be adjusted. You might even be seeing the transaction as successful on Paypal, but the user is not directed back to the order summary page on the Pressero site, and only the error below is shown.
"Error: Invalid Merchant or Merchant doesn't exist!"
In your Paypal Manager account, you'll need to go to Service Settings > Hosted Checkout Pages > Set Up and then to Customize. You must be using Layout C for your hosted payments page to work with the payment method in Pressero.