If you pay via the Payconiq by Bancontact app, then the restrictions imposed by Bancontact itself apply. The current maximum amount per invoice is now fixed at €500. Furthermore, there's also a maximum to take into account if you have connected multiple bank accounts to the Payconiq by Bancontact app: you can pay up to €500 per day per account. Finally, the total amount you of all transactions with all accounts listed in the Bancontact app cannot exceed €500 per day.
If you make the payment with your banking app or by using your card reader, then the maximum amount imposed by your bank for card payments (via Bancontact) applies. Most banks apply a maximum of €2,500 per week.
If the payment is done via Klarna, then no maximum amount is imposed.