Add payment instruction field

What would you put in that field if I may ask? The field is there for manual payments as this gives merchants the ability to provide instructions on how to proceed with the payment? But for PayPal, the order is technically already paid once you complete checkout?

Ex:

Thank you for choosing us for your spare parts. Your order [insert order number] has been successfully confirmed and <strong>will be prepared soon</strong>. You will receive the order summary via email. 

Thank you for choosing us for your spare parts. Your order [insert order number] has been successfully confirmed, and you will receive the order summary via email.
Please remember that it is necessary to proceed with the payment by bank transfer to complete your purchase. 

Little differences but i can add more specific information to customer.
Otherwise I would have to modify the Completion message pane text programmatically.

Thank you.

Please remember that it is necessary to proceed with the payment by bank transfer to complete your purchase.

With PayPal?

Otherwise I would have to modify the Completion message pane text programmatically.

Because you're looking to output different messages based on the payment gateway chosen?

Yes