If you do not tick the 'Payment on due date' box under 'Payment options', your client will be prompted to pay with an immediate transfer, even if you added a payment due date in the payment details.
​
If you added a payment due date without ticking the 'Payment on due date' box, your client will be receiving reminders until they go through the payment process.
If you wish to allow your clients to pay at a further date, it is necessary to tick the 'Payment on due date' box.