I am getting this error message "Unknown error encountered. Please try again!" at the checkout page.  A guest customer will see this error message when making payment through the PayPal or Credit Debit Card payment.  Displayed in red which would suggest to the customer that the transaction was unsuccessful.  However, a test purchase by myself, gets the same message but the transaction goes through.  I believe that this error message is costing me sales.  Can you suggest a fix.

Is there an editable/customisable Tax Invoice that can be emailed to customers on product order (amount owed for ordered product) or on product sale completion (could be sent with the product on shipping).  There is an extension in the marketplace but difficult to tell whether it is suitable and also has not been updated and tested for v1.3.3.