The maximum amount per invoice depends on the payment service provider you will be using to pay.
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 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 pay via Sign2Pay, it is possible to pay invoices of up to €1000, provided that your bank account has been verified (via a 1 cent payment accompanied by a unique code). Without verification, transactions of up to €180 are allowed.