You have a (mobile and/or internet at home) subscription
Then your Viking Points will automatically (partially or fully) be used as payment for your next invoice. Was your invoice partially paid with Viking Points? Then you can pay the remaining amount using a payment method of your choice.

If you have multiple subscriptions on 1 account, your Viking points will be used for the next invoice.
This is an automatic process, so you cannot choose for which payment your Viking points will be used.

Have you recently received an invoice and Viking points shortly afterwards? Then your Viking points will be used for your next invoice and not for the invoice you had already received.

You have a prepaid SIM card
Then you can choose when you use your Viking points. You can choose to set it up automatically when you top up, or you can choose with each top up whether you want to use your Viking points at that moment.